Columbia Acorn Trust

Columbia Acorn® Fund

Summary of the Fund

Investment Objective

Columbia Acorn® Fund (the Fund) seeks long-term capital appreciation.

Fees and Expenses of the Fund

This table describes the fees and expenses that you may pay if you buy, hold and sell shares of the Fund. You may pay other fees, such as brokerage commissions and other fees to financial intermediaries, which are not reflected in the tables and examples below. You may qualify for sales charge discounts if you and members of your immediate family invest, or agree to invest in the future, at least $50,000 in certain classes of shares of eligible funds distributed by Columbia Management Investment Distributors, Inc. (the Distributor). More information is available about these and other sales charge discounts and waivers from your financial intermediary, and can be found in the Choosing a Share Class section beginning on page 23 of the Fund’s prospectus, in Appendix A to the prospectus beginning on page A-1 and in Appendix S to the Statement of Additional Information (SAI) under Sales Charge Waivers beginning on page S-1.

Shareholder Fees (fees paid directly from your investment)
	Class A	Class C	Classes Adv, Inst, Inst2, Inst3, and S
Maximum sales charge (load) imposed on purchases (as a % of offering price)	5.75 %	None	None
Maximum deferred sales charge (load) imposed on redemptions (as a % of the lower of the original purchase price or current net asset value)	1.00 %(a)	1.00 %(b)	None

Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment)
	Class A	Class Adv	Class C	Class Inst	Class Inst2	Class Inst3	Class S
Management fees	0.69 %	0.69 %	0.69 %	0.69 %	0.69 %	0.69 %	0.69 %
Distribution and/or service (12b-1) fees	0.25 %	0.00 %	1.00 %	0.00 %	0.00 %	0.00 %	0.00 %
Other expenses(c)	0.21 %	0.21 %	0.21 %	0.21 %	0.18 %	0.14 %	0.21 %
Total annual Fund operating expenses(d)	1.15 %	0.90 %	1.90 %	0.90 %	0.87 %	0.83 %	0.90 %
Less: Fee waivers and/or expense reimbursements(e)	(0.07 %)	(0.07 %)	(0.07 %)	(0.07 %)	(0.07 %)	(0.08 %)	(0.07 %)
Total annual Fund operating expenses after fee waivers and/or expense reimbursements	1.08 %	0.83 %	1.83 %	0.83 %	0.80 %	0.75 %	0.83 %

(a)

This charge is imposed on certain investments of between $1 million and $50 million redeemed within 18 months after purchase, as follows: 1.00% if redeemed within 12 months after purchase, and 0.50% if redeemed more than 12, but less than 18, months after purchase, with certain limited exceptions.

(b)

This charge applies to redemptions within 12 months after purchase, with certain limited exceptions.

(c)

Other expenses for Class S shares are based on estimated amounts for the Fund’s current fiscal year.

(d)

“Total annual Fund operating expenses” include acquired fund fees and expenses (expenses the Fund incurs indirectly through its investments in other investment companies) and may be higher than the ratio of expenses to average net assets shown in the Financial Highlights section of this prospectus because the ratio of expenses to average net assets does not include acquired fund fees and expenses.

(e)

Columbia Wanger Asset Management, LLC (the Investment Manager) has contractually agreed to waive fees and reimburse certain expenses of the Fund, through April 30, 2025, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of 1.08% for Class A shares, 0.83% for Class Adv shares, 1.83% for Class C shares, 0.83% for Class Inst shares, 0.80% for Class Inst2 shares, 0.75% for Class Inst3 shares and 0.83% for Class S shares. This arrangement may only be amended or terminated with approval from the Fund’s Board of Trustees and the Investment Manager.

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

Example

The following example is intended to help you compare the cost of investing in the Fund with the cost of investing in other mutual funds. The example illustrates the hypothetical expenses that you would incur over the time periods indicated, and assumes that:

■

you invest $10,000 in the applicable class of Fund shares for the periods indicated,

■

your investment has a 5% return each year, and

■

the Fund’s total annual operating expenses remain the same as shown in the Annual Fund Operating Expenses table above.

Class C shares’ 10-year cost examples below reflect the Class C shares' 8-year conversion policy.
Since the waivers and/or reimbursements shown in the Annual Fund Operating Expenses table above expire as indicated in the preceding table, they are only reflected in the 1 year example and the first year of the other examples. Although your actual costs may be higher or lower, based on the assumptions listed above, your costs would be:

	1 year	3 years	5 years	10 years
Class A (whether or not shares are redeemed)	$679	$913	$1,165	$1,886
Class Adv (whether or not shares are redeemed)	$85	$280	$492	$1,101
Class C (assuming redemption of all shares at the end of the period)	$286	$590	$1,020	$2,021
Class C (assuming no redemption of shares)	$186	$590	$1,020	$2,021
Class Inst (whether or not shares are redeemed)	$85	$280	$492	$1,101
Class Inst2 (whether or not shares are redeemed)	$82	$271	$475	$1,066
Class Inst3 (whether or not shares are redeemed)	$77	$257	$453	$1,018
Class S (whether or not shares are redeemed)	$85	$280	$492	$1,101

Portfolio Turnover

The Fund may pay transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when Fund shares are held in a taxable account. These costs, which are not reflected in annual fund operating expenses or in the example, affect the Fund’s performance. During the most recent fiscal year, the Fund’s portfolio turnover rate was 64% of the average value of its portfolio.

Principal Investment Strategies

Under normal circumstances, the Fund invests a majority of its net assets in the common stock of small- and mid-sized companies with market capitalizations generally in the range of market capitalizations in the Russell 2500 Growth Index, the Fund's benchmark, (the Index) at the time of purchase (between $16.6 million and $59.1 billion as of March 31, 2024). The market capitalization range and composition of companies in the Index are subject to change. As such, the size of the companies in which the Fund invests may change. The Fund determines a company’s market capitalization at the time of investment. As long as a majority of its net assets are invested in companies within the market capitalization range of the Index, the Fund may continue to hold and make new investments in a security even if the company’s market capitalization grows beyond the market capitalization of the largest company within the range of the Index or falls below the market capitalization of the smallest company within the Index.

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil). The Investment Manager from time to time emphasizes one or more sectors in selecting the Fund’s investments, including the consumer discretionary, health care, industrials, and information technology sectors.

The Investment Manager typically seeks companies with:

■

A strong business franchise that offers growth potential.

■

Products and services in which the company has a competitive advantage.

■

A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

The Investment Manager may sell a portfolio holding if the security reaches the Investment Manager's price target, if the company has a deterioration of fundamentals, such as failing to meet key operating benchmarks, or if the Investment Manager believes other securities are more attractive. The Investment Manager also may sell a portfolio holding to fund redemptions.

Principal Risks

An investment in the Fund involves risks, including those described below, among others. There is no assurance that the Fund will achieve its investment objective and you may lose money. The value of the Fund’s holdings may decline, and the Fund’s net asset value (NAV) and share price may go down. An investment in the Fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other government agency.

Active Management Risk. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Small- and Mid-Cap Company Securities Risk. Investments in small- and mid-cap companies often involve greater risks than investments in larger, more established companies because small- and mid-cap companies tend to have less predictable earnings and may lack the management experience, financial resources, product diversification and competitive strengths of larger companies. Securities of small- and mid-cap companies may be less liquid and more volatile than the securities of larger companies.

Market Risk. The Fund may incur losses due to declines in the value of one or more securities in which it invests. These declines may be due to factors affecting a particular issuer, or the result of, among other things, political, regulatory, market, economic or social developments affecting the relevant market(s) more generally. In addition, turbulence in financial markets and reduced liquidity in equity, credit and/or fixed income markets may negatively affect many issuers, which could adversely affect the Fund’s ability to price or value hard-to-value assets in thinly traded and closed markets and could cause significant redemptions and operational challenges. Global economies and financial markets are increasingly interconnected, and conditions and events in one country, region or financial market may adversely impact issuers in a different country, region or financial market. These risks may be magnified if certain events or developments adversely interrupt the global supply chain; in these and other circumstances, such risks might affect companies worldwide. As a result, local, regional or global events such as terrorism, war, other conflicts, natural disasters, disease/virus outbreaks and epidemics or other public health issues, recessions, depressions or other events – or the potential for such events – could have a significant negative impact on global economic and market conditions.

Issuer Risk. An issuer in which the Fund invests may perform poorly, and the value of its securities may therefore decline, which may negatively affect the Fund’s performance. Underperformance of an issuer may be caused by poor management decisions, competitive pressures, breakthroughs in technology, reliance on suppliers, labor problems or shortages, corporate restructurings, fraudulent disclosures, natural disasters, military confrontations and actions, war, other conflicts, terrorism, disease/virus outbreaks and epidemics or other events, conditions and factors.

Growth Securities Risk. Growth securities typically trade at a higher multiple of earnings than other types of equity securities. Accordingly, the market values of growth securities may never reach their expected market value and may decline in price. In addition, growth securities, at times, may not perform as well as value securities or the stock market in general, and may be out of favor with investors for varying periods of time. Growth securities may also be sensitive to movements in interest rates.

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

Sector Risk. At times, the Fund may have a significant portion of its assets invested in securities of companies conducting business in an industry or related group of industries within one or more economic sectors, including the consumer discretionary, health care, industrials, and information technology sectors. Companies in the same sector may be similarly affected by economic, regulatory, political or market events or conditions, which may make the Fund vulnerable to unfavorable developments in that industry, group of industries or economic sector.

■

Consumer Discretionary Sector. The Fund is vulnerable to the particular risks that may affect companies in the consumer discretionary sector. Companies in the consumer discretionary sector are subject to certain risks, including fluctuations in the performance of the overall domestic and international economies, interest rate changes, increased competition and consumer confidence. Performance of such companies may be affected by factors including reduced disposable household income, reduced consumer spending, and changing demographics and consumer tastes.

■

Health Care Sector. The Fund is vulnerable to the particular risks that may affect companies in the health care sector. Companies in the health care sector are subject to certain risks, including restrictions on government reimbursement for medical expenses, government approval of medical products and services, competitive pricing pressures, and the rising cost of medical products and services (especially for companies dependent upon a relatively limited number of products or services), among others. Performance of such companies may be affected by factors including government regulation, obtaining and protecting patents (or the failure to do so), product liability and other similar litigation as well as product obsolescence.

■

Industrials Sector. The Fund is vulnerable to the particular risks that may affect companies in the industrials sector. Companies in the industrials sector are subject to certain risks, including changes in supply and demand for their specific product or service and for industrial sector products in general, including decline in demand for such products due to rapid technological developments and frequent new product introduction. Performance of such companies may be affected by factors including government regulation, world events, economic conditions and risks for environmental damage and product liability claims.

■

Information Technology Sector. The Fund is vulnerable to the particular risks that may affect companies in the information technology sector. Companies in the information technology sector are subject to certain risks, including the risk that new services, equipment or technologies will not be accepted by consumers and businesses or will become rapidly obsolete. Performance of such companies may be affected by factors including obtaining and protecting patents (or the failure to do so) and significant competitive pressures, including aggressive pricing of their products or services, new market entrants, competition for market share and short product cycles due to an accelerated rate of technological developments. Such competitive pressures may lead to limited earnings and/or falling profit margins. As a result, the value of their securities may fall or fail to rise. In addition, many information technology sector companies have limited operating histories and prices of these companies’ securities historically have been more volatile than other securities, especially over the short term. Some companies in the information technology sector are facing increased government and regulatory scrutiny and may be subject to adverse government or regulatory action, which could negatively impact the value of their securities.

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

Foreign Securities Risk. Investments in or exposure to securities of foreign companies may involve heightened risks relative to investments in or exposure to securities of U.S. companies. Investing in securities of foreign companies subjects the Fund to the risks associated with an issuer’s (and any of its related companies’) country of organization and places of business operations, including risks related to political, regulatory, economic, social, diplomatic and other conditions or events (including, for example, military confrontations and actions, war, other conflicts, terrorism and disease/virus outbreaks and epidemics) occurring in the country or region, as well as risks associated with less developed custody and settlement practices. Foreign securities may be more volatile and less liquid than securities of U.S. companies, and are subject to the risks associated with potential imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of foreign sub-custodians. Some countries have limited governmental oversight and regulation, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. As a result, there is a risk that the security will not be delivered to the Fund or that payment will not be received. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. In certain non-U.S. markets, an issuer’s securities are blocked from trading for a specified number of days before and, in certain instances, after a shareholder meeting. The blocking period can last up to several weeks. Share blocking may prevent the Fund from buying or selling securities during this period. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile, and may be more susceptible to market manipulation, than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries, and some have a higher risk of currency devaluations. Due to the differences in the nature and quality of financial information of issuers of emerging market securities, including auditing and financial reporting standards, financial information and disclosures about such issuers may be unavailable or, if made available, may be considerably less reliable than publicly available information about other foreign securities.

Performance Information

The following bar chart and table show you how the Fund has performed in the past, and can help you understand the risks of investing in the Fund. Class S shares of the Fund had not commenced operations as of the date of this prospectus and has no performance information to report. The bar chart shows how the Fund’s Class Inst share performance has varied for each full calendar year shown. The table below the bar chart compares the Fund’s returns for the periods shown with those of the Russell 2500 Growth Index.

Any share class, such as Class S shares, without performance to report, would have annual returns substantially similar to Class Inst shares. Except for differences in fees and expenses, all share classes of the Fund would have substantially similar annual returns because all share classes of the Fund invest in the same portfolio of securities.

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

The after-tax returns shown in the Average Annual Total Returns table below are calculated using the highest historical individual U.S. federal marginal income tax rates in effect during the period indicated in the table and do not reflect the impact of state, local or foreign taxes. Your actual after-tax returns will depend on your personal tax situation and may differ from those shown in the table. In addition, the after-tax returns shown in the table do not apply to shares held in tax-advantaged accounts such as 401(k) plans or Individual Retirement Accounts (IRAs). The after-tax returns are shown only for Class Inst shares and will vary for other share classes.

The Fund’s past performance (before and after taxes) is no guarantee of how the Fund will perform in the future. Updated performance information can be obtained by calling toll-free 800.345.6611 or visiting columbiathreadneedleus.com.

Year by Year Total Return (%) as of December 31 Each Year*	Best and Worst Quarterly Returns During the Period Shown in the Bar Chart
	Best	2nd Quarter 2020	28.34%
	Worst	2nd Quarter 2022	-23.57%

Year to Date return as of March 31, 2024: 9.61%

Average Annual Total Returns After Applicable Sales Charges (for periods ended December 31, 2023)

	Share Class Inception Date	1 Year	5 Years	10 Years
Class Inst	06/10/1970
returns before taxes		21.85%	7.60%	6.50%
returns after taxes on distributions		21.85%	4.93%	2.15%
returns after taxes on distributions and sale of Fund shares		12.94%	6.01%	4.27%
Class A returns before taxes	10/16/2000	14.53%	6.05%	5.60%
Class Adv returns before taxes	11/08/2012	21.89%	7.59%	6.48%
Class C returns before taxes	10/16/2000	19.67%	6.50%	5.44%
Class Inst2 returns before taxes	11/08/2012	21.94%	7.62%	6.53%
Class Inst3 returns before taxes	11/08/2012	21.95%	7.68%	6.59%
Russell 2500 Growth Index (reflects no deductions for fees, expenses or taxes)		18.93%	11.43%	8.78%

Fund Management

Investment Manager: Columbia Wanger Asset Management, LLC

Portfolio Management	Title	Role with Fund	Service with the Fund Since
Erika K. Maschmeyer, CFA	Head of Domestic Equity and Director of Domestic Research, Portfolio Manager and Analyst at the Investment Manager	Portfolio Manager or Lead Portfolio Manager since 2019	2016

Prospectus 2024

Columbia Acorn® Fund

Summary of the Fund (continued)

Purchase and Sale of Fund Shares

You may purchase or redeem shares of the Fund on any business day by contacting the Fund in the ways described below:

Online	Regular Mail	Express Mail	By Telephone
columbiathreadneedleus.com/investor/	Columbia Management Investment Services Corp. P.O. Box 219104 Kansas City, MO 64121-9104	Columbia Management Investment Services Corp. c/o SS&C GIDS, Inc. 430 W 7th Street, Suite 219104 Kansas City, MO 64105-1407	800.422.3737

You may purchase shares and receive redemption proceeds by electronic funds transfer, by check or by wire. If you maintain your account with a broker-dealer or other financial intermediary, you must contact that financial intermediary to buy, sell or exchange shares of the Fund through your account with the intermediary.

The minimum initial investment amounts for the share classes offered by the Fund are shown below:

Minimum Initial Investment

Class	Category of eligible account	For accounts other than Systematic Investment Plan accounts (as described in the Fund’s Prospectus)	For Systematic Investment Plan accounts
Classes A & C	All accounts other than IRAs	$2,000	$100
Classes A & C	IRAs	$1,000	$100
Classes Adv & Inst	All eligible accounts	$0, $1,000 or $2,000 depending upon the category of eligible investor	$100
Class Inst2	All eligible accounts	None	N/A
Class Inst3	All eligible accounts	$0, $1,000, $2,000 or $1 million depending upon the category of eligible investor	$100 (for certain eligible investors)

More information about these minimums can be found in the Buying, Selling and Exchanging Shares - Buying Shares section of the prospectus. There is no minimum additional investment for any share class.

Tax Information

The Fund normally distributes net investment income and net realized capital gains, if any, to shareholders. These distributions are generally taxable to you as ordinary income, qualified dividend income or capital gains, unless you are investing through a tax-advantaged account, such as a 401(k) plan or an IRA. If you are investing through a tax-advantaged account, you may be taxed upon withdrawals from that account.

Payments to Broker-Dealers and Other Financial Intermediaries

If you purchase the Fund through a broker-dealer or other financial intermediary (such as a bank), the Fund and its related companies – including the Investment Manager, the Distributor and Columbia Management Investment Services Corp. (the Transfer Agent) – may pay the intermediary for the sale of Fund shares and related services. These payments may create a conflict of interest by influencing the broker-dealer or other intermediary and your financial advisor to recommend the Fund over another investment. These potential conflicts of interest may be heightened with respect to broker-dealers owned by Ameriprise Financial, Inc. (Ameriprise Financial) and/or its affiliates. Ask your financial advisor or visit your financial intermediary's website for more information.

Prospectus 2024

Columbia Acorn® Fund

More Information About the Fund

Investment Objective

Columbia Acorn® Fund (the Fund) seeks long-term capital appreciation. The Fund’s investment objective is not a fundamental policy of the Fund and may be changed by the Fund’s Board of Trustees (the Board) without shareholder approval. There is no assurance the Fund’s objective will be achieved.

Principal Investment Strategies

Columbia Wanger Asset Management, LLC, the Fund's investment adviser (the Investment Manager), believes that stocks of small- and mid-sized companies, which generally are not as well known by financial analysts as larger companies, may offer higher return potential than stocks of larger companies. The Fund invests the majority of its assets in U.S. companies, but also may invest up to 33% of its total assets in foreign companies in developed markets (for example, Japan, Canada and the United Kingdom) and in emerging markets (for example, China, India and Brazil). The Investment Manager from time to time emphasizes one or more sectors in selecting the Fund’s investments, including the consumer discretionary, health care, industrials, and information technology sectors.

The Investment Manager typically seeks companies with:

■

A strong business franchise that offers growth potential.

■

Products and services in which the company has a competitive advantage.

■

A stock price the Investment Manager believes is reasonable relative to the assets and earning power of the company.

Principal Risks

Active Management Risk. The Fund is actively managed and its performance therefore will reflect, in part, the ability of the Investment Manager to select investments and to make investment decisions that will achieve the Fund’s investment objective. The Investment Manager’s active management of the Fund could cause the Fund to underperform its benchmark index and/or other funds with similar investment objectives and/or strategies.

Small- and Mid-Cap Company Securities Risk. Securities of small- and mid-cap companies can, in certain circumstances, have a higher potential for gains than securities of larger, more established companies but may also have more risk. For example, small- and mid-cap companies may be more vulnerable to market downturns and adverse business or economic events than larger companies because they may have more limited financial resources and business operations. Small- and mid-cap companies are also more likely than larger companies to have more

Prospectus 2024

Columbia Acorn® Fund

More Information About the Fund (continued)

limited product lines and operating histories and to depend on smaller and generally less experienced management teams. Securities of small- and mid-cap companies may trade less frequently and in smaller volumes and may be less liquid and fluctuate more sharply in value than securities of larger companies. When the Fund takes significant positions in small- and mid-cap companies with limited trading volumes, the liquidation of those positions, particularly in a distressed market, could be prolonged and result in Fund investment losses that would affect the value of your investment in the Fund. In addition, some small- and mid-cap companies may not be widely followed by the investment community, which can lower the demand for their stocks.

The large-scale invasion of Ukraine by Russia in February 2022 has resulted in sanctions and market disruptions, including declines in regional and global stock markets, unusual volatility in global commodity markets and significant devaluations of Russian currency. The extent and duration of the military action are impossible to predict but could continue to be significant. Market disruption caused by the Russian military action, and any countermeasures or responses thereto (including international sanctions, a downgrade in a country’s credit rating, purchasing and financing restrictions, boycotts, tariffs, changes in consumer or purchaser preferences, cyberattacks and espionage) could continue to have severe adverse impacts on regional and/or global securities and commodities markets, including markets for oil and natural gas. These impacts may include reduced market liquidity, distress in credit markets, further disruption of global supply chains, increased risk of inflation, and limited access to investments in certain international markets and/or issuers. These developments and other related events could negatively impact Fund performance.

■

Prospectus 2024

Columbia Acorn® Fund

More Information About the Fund (continued)

changes, increased competition and consumer confidence. Performance of such companies may be affected by factors including reduced disposable household income, reduced consumer spending, and changing demographics and consumer tastes.

■

Liquidity and Trading Volume Risk. Because the Fund may invest a percentage of its assets in foreign securities, including in emerging markets, it may be subject to the liquidity and trading volume risks associated with international investing, including in emerging markets. Due to market conditions, including uncertainty regarding the price of a security, it may be difficult for the Fund to buy or sell foreign portfolio securities at a desirable time or price, which could result in investment losses and may affect the value of your investment in the Fund. This risk of portfolio illiquidity is heightened with respect to small- and mid-cap securities, generally, and foreign small- and mid-cap securities in particular. The Fund may have to lower the selling price, liquidate other investments, or forego another, more appealing investment opportunity as a result of illiquidity in the markets. The Investment Manager will fair value in good faith any securities it deems to be illiquid under consistently applied procedures established by the Fund’s Board. Market conditions are always changing and vary by country and industry sector, and investing in international markets involves unique risks. Although it is difficult to accurately assess trends in trading volumes in foreign markets, a reduction in trading volumes poses challenges to the Fund. This is particularly so because the Fund focuses on small- and mid-cap companies that usually have lower trading volumes and often takes sizeable positions in portfolio companies. As a result of lower trading volumes, it may take longer to buy or sell these securities, which can exacerbate the Fund’s exposure to volatile markets. The Fund may also be limited in its ability to execute favorable trades in foreign portfolio securities in response to changes in share prices and fundamentals. If the Fund is forced to sell securities to meet redemption requests or other cash needs, or in the case of an event affecting liquidity in a particular market or markets, it may be forced to dispose of those securities under disadvantageous circumstances and at a loss. As the Fund grows in size or, conversely, if it faces significant redemption pressure, these considerations take on increasing significance and may adversely impact performance.

Prospectus 2024

Columbia Acorn® Fund

More Information About the Fund (continued)

Foreign Securities Risk. Investments in or exposure to securities of foreign companies may involve heightened risks relative to investments in or exposure to securities of U.S. companies. For example, foreign markets can be extremely volatile. Foreign securities may also be less liquid, making them more difficult to trade, than securities of U.S. companies so that the Fund may, at times, be unable to sell foreign securities at desirable times or prices. Brokerage commissions, custodial costs and other fees are also generally higher for foreign securities. The Fund may have limited or no legal recourse in the event of default with respect to certain foreign securities, including those issued by foreign governments. In addition, foreign governments may impose withholding or other taxes on the Fund’s income, capital gains or proceeds from the disposition of foreign securities, which could reduce the Fund’s return on such securities. In some cases, such withholding or other taxes could potentially be confiscatory. Other risks include: possible delays in the settlement of transactions or in the payment of income; generally less publicly available information about foreign companies; the impact of economic, political, social, diplomatic or other conditions or events (including, for example, military confrontations and actions, war, other conflicts, terrorism and disease/virus outbreaks and epidemics), possible seizure, expropriation or nationalization of a company or its assets or the assets of a particular investor or category of investors; accounting, auditing and financial reporting standards that may be less comprehensive and stringent than those applicable to domestic companies; the imposition of economic and other sanctions against a particular foreign country, its nationals or industries or businesses within the country; and the generally less stringent standard of care to which local agents may be held in the local markets. In addition, it may be difficult to obtain reliable information about the securities and business operations of certain foreign issuers. Governments or trade groups may compel local agents to hold securities in designated depositories that are not subject to independent evaluation. The less developed a country’s securities market is, the greater the level of risks. Economic sanctions may be, and have been, imposed against certain countries, organizations, companies, entities and/or individuals. Economic sanctions and other similar governmental actions could, among other things, effectively restrict or eliminate the Fund’s ability to purchase or sell securities, and thus may make the Fund’s investments in such securities less liquid or more difficult to value. In addition, as a result of economic sanctions, the Fund may be forced to sell or otherwise dispose of investments at inopportune times or prices, which could result in losses to the Fund and increased transaction costs. These conditions may be in place for a substantial period of time and enacted with limited advance notice to the Fund. The risks posed by sanctions against a particular foreign country, its nationals or industries or businesses within the country may be heightened to the extent the Fund invests significantly in the affected country or region or in issuers from the affected country that depend on global markets. Additionally, investments in certain countries may subject the Fund to a number of tax rules, the application of which may be uncertain. Countries may amend or revise their existing tax laws, regulations and/or procedures in the future, possibly with retroactive effect. Changes in or uncertainties regarding the laws, regulations or procedures of a country could reduce the after-tax profits of the Fund, directly or indirectly, including by reducing the after-tax profits of companies located in such countries in which the Fund invests, or result in unexpected tax liabilities for the Fund.

Operational and Settlement Risks of Foreign Securities. The Fund’s foreign securities are generally held outside the United States in the primary market for the securities in the custody of certain eligible foreign banks and trust companies (foreign sub-custodians), as permitted under the Investment Company Act of 1940 (the 1940 Act). Settlement practices for foreign securities may differ from those in the United States. Some countries have limited governmental oversight and regulation of industry practices, stock exchanges, depositories, registrars, brokers and listed companies, which increases the risk of corruption and fraud and the possibility of losses to the Fund. In particular, under certain circumstances, foreign securities may settle on a delayed delivery basis, meaning that the Fund may be required to make payment for securities before the Fund has actually received delivery of the securities or deliver securities prior to the receipt of payment. Typically, in these cases, the Fund will receive evidence of ownership in accordance with the generally accepted settlement practices in the local market entitling the Fund to delivery or payment at a future date, but there is a risk that the security will not be delivered to the Fund or that payment will not be received, although the Fund and its foreign sub-custodians take reasonable precautions to mitigate this risk. Losses can also result from lost, stolen or counterfeit securities; defaults by brokers and banks; failures or defects of the settlement system; or poor and improper record keeping by registrars and issuers.

Share Blocking. Share blocking refers to a practice in certain foreign markets under which an issuer’s securities are blocked from trading at the custodian or sub-custodian level for a specified number of days before and, in certain instances, after a shareholder meeting where a vote of shareholders takes place. The blocking period can last up to

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several weeks. Share blocking may prevent the Fund from buying or selling securities during this period, because during the time shares are blocked, trades in such securities will not settle. It may be difficult or impossible to lift blocking restrictions, with the particular requirements varying widely by country. As a consequence of these restrictions, the Investment Manager, on behalf of the Fund, may abstain from voting proxies in markets that require share blocking.

Emerging Market Securities Risk. Securities issued by foreign governments or companies in emerging market countries are more likely to have greater exposure to the risks of investing in foreign securities that are described in Foreign Securities Risk. In addition, emerging market countries are more likely to experience instability resulting, for example, from rapid changes or developments in social, political, economic or other conditions. Their economies are usually less mature and their securities markets are typically less developed with more limited trading activity (i.e., lower trading volumes and less liquidity) than more developed countries. Emerging market securities tend to be more volatile, and may be more susceptible to market manipulation, than securities in more developed markets. Many emerging market countries are heavily dependent on international trade and have fewer trading partners, which makes them more sensitive to world commodity prices and economic downturns in other countries. Some emerging market countries have a higher risk of currency devaluations, and some of these countries may experience periods of high inflation or rapid changes in inflation rates and may have hostile relations with other countries. Due to the differences in the nature and quality of financial information of issuers of emerging market securities, including auditing and financial reporting standards, financial information and disclosures about such issuers may be unavailable or, if made available, may be considerably less reliable than publicly available information about other foreign securities.

Additional Investment Strategies and Policies

This section describes certain investment strategies and policies that the Fund may utilize in pursuit of its investment objective and some additional factors and risks involved with investing in the Fund. For information about risks associated with changing levels of Fund distributions, see the Fund’s SAI.

Changing the Fund’s Investment Objective and Policies

The Fund’s investment objective and non-fundamental investment policies (including its principal and additional investment strategies) can be changed by the Board without shareholder approval, but may require notice to shareholders in certain instances. The Fund’s fundamental investment policies, as identified in the SAI, may be changed only with Board and shareholder approval in accordance with the voting requirements of the 1940 Act. For additional information about changing the Fund’s fundamental and non-fundamental investment policies, see the SAI.

Investment Guidelines

As a general matter, and except as specifically described in the discussion of the Fund's principal investment strategies in this prospectus, whenever an investment policy or limitation states a percentage of the Fund's assets that may be invested in any security or other asset or sets forth a policy regarding an investment standard, compliance with that percentage limitation or standard will be determined based on the characteristics of a company at the time of initial purchase, and subsequent changes in a characteristic are not taken into account.

Holding Other Kinds of Investments

The Fund may hold other investments that are not part of its principal investment strategies. These investments and their risks are described below and/or in the SAI. The Fund may choose not to invest in certain securities described in this prospectus and in the SAI, although it has the ability to do so. Information on the Fund’s holdings can be found in the Fund’s shareholder reports or by visiting columbiathreadneedleus.com.

Transactions in Derivatives

The Fund may enter into derivative transactions or otherwise have exposure to derivative transactions through underlying investments. The Fund expects to enter into such transactions primarily in the form of stock index futures in order to obtain exposure to the overall sector of the market represented by a particular stock index or to invest the Fund’s uninvested cash balances in a manner that the Investment Manager believes aligns more closely with the

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Fund’s investment strategy than simply holding short-term fixed income investments or cash equivalents (known as cash equitization). Derivatives are financial contracts whose values are, for example, based on (or “derived” from) traditional securities (such as a stock or bond), assets (such as a commodity like gold or a foreign currency), reference rates (such as the Secured Overnight Financing Rate (commonly known as SOFR)) or market indices (such as the Standard & Poor’s 500® Index). The use of derivatives is a highly specialized activity which involves investment techniques and risks different from those associated with ordinary portfolio securities transactions. Derivatives involve special risks and may result in losses or may limit the Fund’s potential gain from favorable market movements. Derivative strategies typically expose the Fund to gains and losses in excess of the amount actually invested in the derivative. This characteristic of a derivative (known as "leverage") may exaggerate a loss, potentially causing the Fund to lose more money than it would have lost had it invested in the underlying security or other asset directly. The values of derivatives may move in unexpected ways, especially in unusual (including distressed) market conditions, and may result in increased volatility in the value of the derivative and/or the Fund’s shares, among other consequences. The use of derivatives may also increase the amount of taxes payable by shareholders holding shares in a taxable account. Other risks arise from the Fund’s potential inability to terminate or to sell derivative positions. A liquid secondary market may not always exist for the Fund’s derivative positions at times when the Fund might wish to terminate or to sell such positions. Over-the-counter instruments (investments not traded on an exchange) may be illiquid, and transactions in derivatives traded in the over-the-counter market are subject to the risk that the other party will not meet its obligations. The use of derivatives also involves the risks of mispricing or improper valuation and that changes in the value of the derivative may not correlate perfectly with the underlying security, asset, reference rate or index. The Fund also may not be able to find a suitable derivative transaction counterparty, and thus may be unable to engage in derivative transactions when it is deemed favorable to do so, or at all. U.S. federal legislation has been enacted that provides for new clearing, margin, reporting and registration requirements for participants in the derivatives market. These changes could restrict and/or impose significant costs or other burdens upon the Fund’s participation in derivatives transactions. The U.S. government and the European Union (and some other jurisdictions) have enacted regulations and similar requirements that prescribe clearing, margin, reporting and registration requirements for participants in the derivatives market. These requirements are evolving and their ultimate impact on the Fund remains unclear, but such impact could include restricting and/or imposing significant costs or other burdens upon the Fund’s participation in derivatives transactions. Additionally, regulations governing the use of derivatives by registered investment companies, such as the Fund, require, among other things, that a fund that invests in derivative instruments beyond a specified limited amount to apply a value-at-risk-based limit to its portfolio and establish a comprehensive derivatives risk management program. As of the date of this prospectus, the Fund is not required to maintain a comprehensive derivatives risk management program given its more limited use of derivatives. For more information on the risks of derivative investments and strategies, see the SAI.

Affiliated Fund Investing

The Investment Manager and its affiliate, Columbia Management Investment Advisers, LLC (Columbia Management), serve as investment advisers to mutual funds and ETFs using the Columbia brand (Columbia Funds), including mutual funds that are structured as “fund-of-funds”, and provides asset-allocation services to (i) shareholders by investing in shares of other Columbia Funds, which may include the Fund (collectively referred to in this section as Underlying Funds), and (ii) discretionary managed accounts (collectively referred to as affiliated products) that invest exclusively in Underlying Funds. These affiliated products, individually or collectively, may own a significant percentage of the outstanding shares of one or more Underlying Funds, and the Investment Manager and Columbia Management seek to balance potential conflicts of interest between the affiliated products and the Underlying Funds in which they invest. The affiliated products’ investment in the Underlying Funds may have the effect of creating economies of scale, possibly resulting in lower expense ratios for the Underlying Funds, because the affiliated products may own substantial portions of the shares of Underlying Funds. However, redemption of Underlying Fund shares by one or more affiliated products could cause the expense ratio of the Underlying Fund to increase, as its fixed costs would be spread over a smaller asset base. Because of large positions of certain affiliated products, the Underlying Funds may experience relatively large inflows and outflows of cash due to affiliated products’ purchases and sales of Underlying Fund shares. Although the Investment Manager or Columbia Management may seek to minimize the impact of these

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transactions where possible, for example, by structuring them over a reasonable period of time or through other measures, Underlying Funds may experience increased expenses as they buy and sell portfolio securities to manage the cash flow effect related to these transactions. Further, when the Investment Manager or Columbia Management structures transactions over a reasonable period of time in order to manage the potential impact of the buy and sell decisions for the affiliated products, those affiliated products, including funds-of-funds, may pay more or less (for purchase activity), or receive more or less (for redemption activity), for shares of the Underlying Funds than if the transactions were executed in one transaction. In addition, substantial redemptions by affiliated products within a short period of time could require the Underlying Fund to liquidate positions more rapidly than would otherwise be desirable, which may have the effect of reducing or eliminating potential gain or causing it to realize a loss. In order to meet such redemptions, the Underlying Fund may be forced to sell its liquid (or more liquid) positions, leaving the Underlying Fund holding, post-redemption, a relatively larger position in illiquid investments (i.e., any investment that the Fund reasonably expects cannot be sold or disposed of in current market conditions in seven calendar days or less without the sale or disposition significantly changing the market value of the investment) or less liquid securities. Substantial redemptions may also adversely affect the ability of the Underlying Fund to implement its investment strategy. The Investment Manager or Columbia Management also has a conflict of interest in determining the allocation of affiliated products’ assets among the Underlying Funds, as it earns different fees from the various Underlying Funds.

Investing in Money Market Funds

The Fund may invest cash, including cash collateral received in connection with its securities lending program, in shares of money market funds, including funds advised by affiliates of the Investment Manager. These funds are not insured or guaranteed by the Federal Deposit Insurance Corporation (FDIC) or any other government agency. The Fund and its shareholders indirectly bear a portion of the expenses of any money market fund or other fund in which the Fund may invest.

Lending of Portfolio Securities

The Fund may lend portfolio securities to broker-dealers or other financial intermediaries on a fully collateralized basis in order to earn additional income. The Fund may lose money from securities lending if, for example, the borrower does not return a loaned security and the proceeds from sale of the collateral are not sufficient to replace the borrowed security. The Fund invests collateral in a government money market fund.

Investing Defensively

The Fund may from time to time take temporary defensive investment positions that are inconsistent with the Fund’s principal investment strategies in attempting to respond to adverse market, economic, political, social or other conditions. These investment positions may include, without limitation, investing some or all of its assets in money market instruments or shares of affiliated or unaffiliated money market funds or holding some or all of its assets in cash or cash equivalents. The Fund may take such defensive investment positions for as long a period as the Investment Manager deems necessary. During these times, the Investment Manager may make frequent portfolio changes, which could result in increased trading expenses and taxes, and decreased Fund performance.

Portfolio Holdings Disclosure

A description of the Fund’s policies and procedures with respect to the disclosure of Fund portfolio securities is available in the SAI. The Fund discloses its portfolio holdings on the Columbia Funds’ website, columbiathreadneedleus.com, as described below. Once posted, the portfolio holdings information will remain available on the website until at least the date on which such Fund files a Form N-CSR or publicly available Form N-PORT (forms filed with the SEC that include portfolio holdings information) for the period that includes the date as of which the information is current.

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The Fund considers changes in its portfolio holdings to be confidential information. Consequently, Fund policy generally permits the disclosure of portfolio holdings information only after a certain amount of time has passed. The Fund’s complete portfolio holdings are disclosed approximately 30 to 40 days after each month-end. The top 15 holdings may be available sooner, approximately 15 calendar days after each month-end. Purchases and sales of portfolio securities can take place at any time, so the portfolio holdings information available on the website may not always be current.

Cybersecurity

The Fund, like all companies, may be susceptible to operational and information security risks, including the risk of cyber attacks or failures. As a result, the Fund or its service providers, or the issuers of securities in which the Fund invests, may experience disruptions in business operations that may potentially result in financial losses, the inability of the Fund or Fund shareholders to transact business, violations of applicable privacy and other laws, regulatory fines, penalties, reputational damage, reimbursement or other compensation costs, and/or additional compliance costs. The Fund and its shareholders could be negatively impacted as a result. See the discussion of Cybersecurity Breaches and Technology and Related Systems Failure Risk in the SAI for further information.

Use of Benchmarks

Benchmarks are indices that provide some comparative guidance in assessing the Fund’s performance. The Investment Manager selects the benchmark(s) it believes provide meaningful comparisons for the Fund. However, there may be different or additional indices that more closely reflect the market sectors in which the Fund invests. The Fund’s benchmark(s) may change from time to time. The benchmark included in this prospectus is intended only as a guidepost for your assessment of the Fund’s performance.

FUNDamentals

Portfolio Holdings Versus the Benchmarks

The Fund does not limit its investments to the securities within its benchmarks, and accordingly the Fund's holdings may diverge significantly from those of the benchmarks selected by the Investment Manager. In addition, the Fund may invest in securities outside the industry and geographic sectors represented in its benchmarks. The Fund's weightings in individual securities, and in industry and geographic sectors, may also vary considerably from those of its benchmarks.

eDelivery and Mailings to Households

In order to reduce shareholder expenses, the Fund may mail only one copy of the Fund’s prospectus and each annual and semiannual report to those addresses shared by two or more accounts. If you wish to receive separate copies of these documents, call 800.345.6611 or, if your shares are held through a financial intermediary, contact your intermediary directly. Additionally, you may elect to enroll in eDelivery to receive electronic versions of these documents, as well as quarterly statements and supplements, by logging into your account at columbiathreadneedleus.com/investor/.

Additional Information on Portfolio Turnover

A mutual fund that replaces, or turns over, more than 100% of its investments in a year may be considered to have a high portfolio turnover rate. A high portfolio turnover rate can generate larger distributions of short-term capital gains to shareholders, which for individuals are generally taxable at higher rates than long-term capital gains for U.S. federal income tax purposes. Also, a high portfolio turnover rate can mean higher brokerage commissions and other transaction costs, which could reduce a fund’s returns. In general, the greater the volume of buying and selling by a fund, the greater the impact that brokerage commissions and other transaction costs will have on its returns. The Fund may sell securities regardless of how long they’ve been held. See the Financial Highlights section of this prospectus for the Fund's portfolio turnover rates for the past five years.

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Cash Flows

The timing and magnitude of cash inflows from investors buying Fund shares could prevent the Fund from always being fully invested. Conversely, the timing and magnitude of cash outflows to shareholders redeeming Fund shares could require the Fund to sell portfolio securities at less than opportune times or to hold ready reserves of uninvested cash in amounts larger than might otherwise be the case to meet shareholder redemptions. Either situation could adversely impact the Fund’s performance.

Understanding Annual Fund Operating Expenses

The Fund’s annual operating expenses, as presented in the Annual Fund Operating Expenses table in the Fees and Expenses of the Fund section of this prospectus, generally are based on expenses incurred during the Fund’s most recently completed fiscal year, may vary by share class and are expressed as a percentage (expense ratio) of the Fund’s average net assets during that fiscal year. The expense ratios reflect the Fund’s fee arrangements as of the date of this prospectus and, unless indicated otherwise, are based on expenses incurred during the Fund’s most recent fiscal year. The Fund’s assets will fluctuate, but unless indicated otherwise in the Annual Fund Operating Expenses table, no adjustments have been or will be made to the expense ratios to reflect any differences in the Fund’s average net assets between the most recently completed fiscal year and the date of this prospectus or a later date. In general, the Fund’s expense ratios will increase as its net assets decrease, such that the Fund’s actual expense ratios may be higher than the expense ratios presented in the Annual Fund Operating Expenses table if assets fall. Any commitment by the Investment Manager and/or its affiliates to waive fees and/or cap (reimburse) expenses is expected, in part, to limit the impact of any increase in the Fund’s expense ratios that would otherwise result because of a decrease in the Fund’s assets in the current fiscal year. The Fund’s annual operating expenses are comprised of (i) investment management fees, (ii) distribution and/or service fees, and (iii) other expenses. Management fees do not vary by class, but distribution and/or service fees and other expenses may vary by class. Please see Fee Waiver/Expense Reimbursement Arrangements below for information about commitments made by the Investment Manager and/or its affiliates during the performance period covered by this prospectus.

FUNDamentals

Other Expenses

“Other expenses” consist of the fees the Fund pays to its administrator, custodian, transfer agent, auditors, lawyers and trustees, costs relating to compliance and miscellaneous expenses. Generally, these expenses are allocated on a pro rata basis across all share classes. These expenses include certain sub-transfer agency and shareholder servicing fees. Transfer agency fees and certain shareholder servicing fees, however, are class specific. They differ by share class because the shareholder services provided to each share class may be different. Accordingly, the differences in “other expenses” among share classes are primarily the result of the different transfer agency and shareholder servicing fees applicable to each share class. For more information on these fees, see Choosing a Share Class — Financial Intermediary Compensation.

Fee Waiver/Expense Reimbursement Arrangements

The Investment Manager has contractually agreed to waive fees and reimburse certain expenses of the Fund through April 30, 2025, so that ordinary operating expenses (excluding transaction costs and certain other investment-related expenses, interest and fees on borrowings, and expenses associated with the Fund’s investment in other investment companies, if any) do not exceed the annual rates of:

Columbia Acorn Fund
Class A	1.08%
Class Adv	0.83%
Class C	1.83%
Class Inst	0.83%

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Columbia Acorn Fund
Class Inst2	0.80%
Class Inst3	0.75%
Class S	0.83%

This arrangement may only be amended or terminated with approval from the Board and the Investment Manager.

Effect of Fee Waivers and/or Expense Reimbursements on Past Performance. The Fund’s returns shown in the Performance Information section of this prospectus reflect the effect of fee waivers and/or reimbursements of Fund expenses by the Investment Manager and/or its affiliates that were in place during the performance period shown in the Performance Information section of this prospectus. Without such fee waivers/expense reimbursements, the Fund’s returns might have been lower.

Board of Trustees

The Fund is governed by the Board. More than 75% of the Fund's Trustees are independent (Independent Trustees), meaning that they are not “interested persons” of the Fund, as defined in the 1940 Act. The Independent Trustees bring backgrounds in business and academia to their task of working with the Fund's officers to establish the Fund's policies and oversee its activities. Among the Trustees' responsibilities are: selecting the investment adviser for the Fund; approving the advisory agreement; reviewing other contracts; approving investment policies; monitoring Fund operations, performance, compliance and costs; and nominating or selecting new Trustees.

Each Trustee serves the Fund for an indefinite term until his or her retirement, resignation, death or removal in accordance with the organizational documents of Columbia Acorn Trust (the Trust). The Trust's Bylaws generally require that Trustees retire at the end of the calendar year in which they attain the age of 75 years. The last meeting to elect Trustees was held on February 27, 2015. Any Trustee may be removed at a shareholders' meeting by a vote representing two-thirds of the net asset value of all shares of the Columbia Acorn family of funds (Columbia Acorn Funds). The mailing address for the Trustees and officers is 71 S. Wacker Drive, Suite 2500, Chicago, Illinois 60606.

For more detailed information about the Board, please refer to the SAI.

Primary Service Provider Contracts

The Fund enters into contractual arrangements (Service Provider Contracts) with various parties, including, among others, the Investment Manager, the Distributor, the Transfer Agent and the Fund’s custodian. The Fund’s Service Provider Contracts are solely among the parties thereto. Shareholders are not parties to, or intended to be third-party beneficiaries of, any Service Provider Contracts. Further, this prospectus, the SAI and any Service Provider Contracts are not intended to give rise to any agreement, duty, special relationship or other obligation between the Fund and any investor, or give rise to any contractual, tort or other rights in any individual shareholder, group of shareholders or other person, including any right to assert a fiduciary or other duty, enforce the Service Provider Contracts against the parties or to seek any remedy thereunder, either directly or on behalf of the Fund. Nothing in the previous sentence should be read to suggest any waiver of any rights under federal or state securities laws.

The Investment Manager, which also serves as the Fund's administrator (the Administrator), Columbia Management, which serves as the Fund's sub-administrator pursuant to an agreement with the Investment Manager (the Sub-Administrator), the Distributor, and the Transfer Agent are all affiliates of Ameriprise Financial. They and their affiliates currently provide key services, including investment advisory, administration, distribution, shareholder servicing and transfer agency services, to the Fund and various other funds, including the Columbia Funds, and are paid for providing these services. These service relationships are described below.

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The Investment Manager

Columbia Wanger Asset Management, LLC is located at 71 S. Wacker Drive, Suite 2500, Chicago, Illinois 60606. As of March 31, 2024, the Investment Manager had assets under management of approximately $6.5 billion. The Investment Manager is a registered investment adviser and wholly-owned subsidiary of Columbia Management, which is a wholly-owned subsidiary of Ameriprise Financial. In addition to serving as an investment adviser to mutual funds, the Investment Manager acts as an investment manager for other private funds and institutional accounts.

Subject to oversight by the Board, the Investment Manager manages the day-to-day operations of the Fund, determining what securities and other investments the Fund should buy or sell and executing portfolio transactions. The Investment Manager may use the research and other capabilities of its affiliates and third parties in managing the Fund’s investments.

The Fund pays the Investment Manager a fee for its investment advisory services. The fee is calculated as a percentage of the daily net assets of the Fund and is paid monthly. For the Fund’s most recent fiscal year, aggregate fees paid to the Investment Manager by the Fund amounted to 0.69% of average daily net assets of the Fund, before any applicable reimbursements.

A discussion regarding the basis for the Board’s approval of the renewal of the Fund’s investment management services agreement with the Investment Manager is available in the Fund’s semiannual report to shareholders for the fiscal period ended June 30, 2023.

The Investment Manager and its investment advisory affiliates, including Columbia Management and Threadneedle International Limited (Threadneedle) (Participating Affiliates), around the world may coordinate in providing services to their clients. From time to time, the Investment Manager may engage its Participating Affiliates to provide a variety of services such as investment research, investment monitoring, trading, and discretionary investment management (including portfolio management and proxy voting) to certain accounts managed by the Investment Manager, including the Fund. These Participating Affiliates will provide services to the Fund and other accounts of the Investment Manager either pursuant to delegation agreements, personnel-sharing agreements or similar inter-company or other arrangements or relationships and the Fund will pay no additional fees and expenses as a result of any such arrangements or relationships. These Participating Affiliates, like the Investment Manager, are direct or indirect subsidiaries of Ameriprise Financial and are registered with the appropriate respective regulators in their home jurisdictions and, where required, the SEC and the Commodity Futures Trading Commission in the United States.

Pursuant to some of these arrangements or relationships, certain personnel of Columbia Management and other Participating Affiliates serve as “associated persons” or officers of the Investment Manager and, in this capacity, subject to the oversight and supervision of the Investment Manager and consistent with the investment objectives, policies and limitations set forth in this prospectus and the Fund's SAI, and with the Investment Manager’s and the Fund's compliance policies and procedures, provide services to the Fund.

Portfolio Manager

Information about the portfolio manager primarily responsible for overseeing the Fund’s investments is shown below. The SAI provides additional information about the portfolio manager, including information relating to compensation, other accounts managed by the portfolio manager, and ownership by the portfolio manager of Fund shares.

Portfolio Management	Title	Role with Fund	Service with the Fund Since
Erika K. Maschmeyer, CFA	Head of Domestic Equity and Director of Domestic Research, Portfolio Manager and Analyst at the Investment Manager	Portfolio Manager or Lead Portfolio Manager since 2019	2016

Ms. Maschmeyer has been associated with the Investment Manager as an investment professional since 2016. Ms. Maschmeyer heads the Investment Manager's U.S. equity research efforts and has been a Vice President of the Trust since March 2020. Prior to joining the Investment Manager, Ms. Maschmeyer was a research analyst at an investment advisory firm where she was responsible for U.S. consumer discretionary/staples investments. Ms. Maschmeyer began her investment career in 2001 and earned a B.A. from Denison University and an M.B.A from the University of Chicago.

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The Administrator

Columbia Wanger Asset Management, LLC is responsible for overseeing the administrative operations of the Fund, including the general supervision of the Fund’s operations, coordination of the Fund’s service providers, and the provision of office facilities and related clerical and administrative services. The Administrator retains the Sub-Administrator to perform many of these services (including fund accounting), and pays a fee for the services of the Sub-Administrator. The Fund pays the Administrator a fee for its services, plus certain out-of-pocket expenses. The fee is calculated as an annual percentage of the Trust’s aggregate average daily net assets and is paid monthly, as follows:

Annual Administration Fee, as a % of Aggregate Daily Net Assets of the Trust:
Up to $8 billion	0.050%
$8 billion to $16 billion	0.040%
$16 billion to $35 billion	0.030%
$35 billion to $45 billion	0.025%
$45 billion and over	0.015%

The Distributor

Shares of the Fund are distributed by Columbia Management Investment Distributors, Inc., which is located at 290 Congress Street, Boston, MA 02210. The Distributor is a registered broker-dealer and an indirect, wholly-owned subsidiary of Ameriprise Financial. The Distributor and its affiliates may pay commissions, distribution and service fees and/or other compensation to entities, including Ameriprise Financial affiliates, for selling shares and providing services to investors.

The Transfer Agent

Columbia Management Investment Services Corp. is a registered transfer agent and wholly-owned subsidiary of Ameriprise Financial. The Transfer Agent is located at 290 Congress Street, Boston, MA 02210, and its responsibilities include processing purchases, redemptions and exchanges of Fund shares, calculating and paying distributions, maintaining shareholder records, preparing account statements and providing customer service. The Transfer Agent has engaged SS&C GIDS, Inc. to provide various shareholder or “sub-transfer agency” services. In addition, the Transfer Agent enters into agreements with various financial intermediaries through which you may hold Fund shares, pursuant to which the Transfer Agent pays these financial intermediaries for providing certain shareholder services. Depending on the type of account, the Fund pays the Transfer Agent a per account fee for each open account and compensates the Transfer Agent for certain out-of-pocket expenses, including certain payments to financial intermediaries through which shares are held. In addition, subject to certain limitations, the Fund compensates the Transfer Agent for payments made to financial intermediaries for the shareholder services that the intermediaries provide, in amounts that vary by share class and with the type of intermediary and the type of shareholder services provided. The Transfer Agent is contractually obligated to oversee such intermediaries for the purpose of reasonably assuring their compliance with applicable law and the terms of their agreements to provide the shareholder services. For more information about Shareholder Services fees, see Choosing a Share Class — Financial Intermediary Compensation.

Other Roles and Relationships of Ameriprise Financial and its Affiliates — Certain Conflicts of Interest

This section describes certain actual and potential conflicts of interest that arise from the financial services activities of Ameriprise Financial and its affiliates. Ameriprise Financial is a major financial services company, engaged in a broad range of financial activities beyond the fund-related activities of the Investment Manager, including: insurance, broker-dealer (sales and trading), asset management and financial services. As a consequence of these activities, Ameriprise Financial or its affiliates may have interests arising from their other business lines that conflict with the interests of the Fund. These activities may also, from time to time, place certain investment constraints on the management of the Fund that might not otherwise arise.

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As described in More Information About the Fund - Primary Service Provider Contracts, the Investment Manager, Administrator, Sub-Administrator, Distributor and Transfer Agent are all affiliates of Ameriprise Financial. In addition to the services that they provide to the Fund, they also provide substantially similar services (for which they are compensated) to other clients and customers, including the Columbia Funds. Ameriprise Financial and its other affiliates may also provide services (for which they are compensated) to the Fund, other Columbia Funds or other clients and customers.

Examples of activities that could lead to conflicts of interest and/or impose limitations that could affect the Fund include the following:

■

the Investment Manager and other Ameriprise Financial affiliates may receive compensation and other benefits related to the management/administration of the Fund and the other Columbia Funds and the sale of their shares;

■

there may be competition for limited investment opportunities that must be allocated among the Fund, the other Columbia Funds and other clients and customers of the Investment Manager (which can include affiliates of the Investment Manager) that may have the same or similar investment objectives as the Fund;

■

management of the Fund may diverge from other Columbia Funds or other clients and customers of the Investment Manager or Ameriprise Financial affiliates, for example, advice given to the Fund may differ from, or conflict with, advice given to other funds or accounts;

■

there may be regulatory or investment restrictions imposed on the investment activities of the Investment Manager arising from the activities or holdings of other Columbia Funds or other clients or customers of the Investment Manager or Ameriprise Financial and its affiliates, for example, caps on the aggregate amount of certain types of investments or in holdings of particular portfolio companies that may be made by affiliated entities, including the Fund;

■

Ameriprise Financial or its affiliates may have potentially conflicting relationships with companies and other entities in which the Fund invests; and

■

there may be regulatory and other restrictions relating to the sharing of information between Ameriprise Financial and its affiliates, including the Investment Manager, for example, if an affiliated entity were in possession of non-public information, the Investment Manager might be prohibited by law from using that information in connection with the management of the Fund.

The Investment Manager and Ameriprise Financial have adopted various policies and procedures that are intended to identify, monitor and address conflicts of interest. However, there is no absolute assurance that these policies, procedures and disclosures will be effective.

Additional information about Ameriprise Financial and the types of conflicts of interest and other matters referenced above is set forth in the Investment Advisory and Other Services – Other Roles and Relationships of Ameriprise Financial and Affiliates – Certain Conflicts of Interest section of the SAI. Investors in the Fund should carefully review these disclosures and consult with their financial advisor if they have any questions.

Certain Legal Matters

Ameriprise Financial and certain of its affiliates are involved in the normal course of business in legal proceedings which include regulatory inquiries, arbitration and litigation, including class actions concerning matters arising in connection with the conduct of their activities as part of a diversified financial services firm. Ameriprise Financial believes that the Fund is not currently the subject of, and that neither Ameriprise Financial nor any of its affiliates are the subject of, any pending legal, arbitration or regulatory proceedings that are likely to have a material adverse effect on the Fund or the ability of Ameriprise Financial or its affiliates to perform under their contracts with the Fund. Information regarding certain pending and settled legal proceedings may be found in the Fund’s shareholder reports and in the SAI. Additionally, Ameriprise Financial is required to make quarterly (10-Q), annual (10-K) and, as necessary, 8-K filings with the SEC on legal and regulatory matters that relate to Ameriprise Financial and its affiliates. Copies of these filings may be obtained by accessing the SEC website at sec.gov.

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class

The Funds

The Columbia Acorn Funds and the other Columbia Funds generally share the same policies and procedures for investor services, as described below. Together, the Fund and the other Columbia Acorn Funds, along with the Columbia Funds, are referred to herein as the Funds.

Funds Contact Information

Additional information about the Funds, including sales charges and other class features and policies, can be obtained, free of charge, at columbiathreadneedleus.com,* by calling toll-free 800.345.6611, or by writing (regular mail) to Columbia Management Investment Services Corp., P.O. Box 219104, Kansas City, MO 64121-9104 or (express mail) Columbia Management Investment Services Corp., c/o SS&C GIDS, Inc., 430 W 7th Street, Ste 219104, Kansas City, MO 64105-1407.

The website references in this prospectus are inactive links and information contained in or otherwise accessible through the referenced websites does not form a part of this prospectus.

FUNDamentals

Financial Intermediaries

The term “financial intermediary” refers to the selling and servicing agents that are authorized to sell and/or service shares of the Funds. Financial intermediaries include broker-dealers and financial advisors as well as firms that employ broker-dealers and financial advisors, including, for example, brokerage firms, banks, investment advisers, third party administrators and other firms in the financial services industry.

Omnibus Accounts

The term “omnibus account” refers to a financial intermediary’s account with the Fund (held directly through the Transfer Agent) that represents the combined holdings of, and transactions in, Fund shares of one or more clients of the financial intermediary (beneficial Fund shareholders). Omnibus accounts are held in the name of the financial intermediaries and not in the name of the beneficial Fund shareholders invested in the Fund through omnibus accounts.

Retirement Plans and Omnibus Retirement Plans

The term “retirement plan” refers to retirement plans created under Sections 401(a), 401(k), 457 and 403(b) of the Internal Revenue Code of 1986, as amended (the Code), and non-qualified deferred compensation plans governed by Section 409A of the Code and similar plans, but does not refer to individual retirement plans, such as traditional IRAs and Roth IRAs. The term “omnibus retirement plan” refers to a retirement plan that has a plan-level or omnibus account with the Transfer Agent.

Networked Accounts

Networking, offered by the Depository Trust & Clearing Corporation’s Wealth Management Services (WMS), is the industry standard IT system for mutual fund account reconciliation and dividend processing.

Summary of Share Class Features

Each share class has its own investment eligibility criteria, cost structure and other features. You may not be eligible to invest in every share class. Your financial intermediary may not make every share class available or may cease to make available one or more share classes of the Fund. The share class you select through your financial intermediary may have higher fees and/or sales charges than other classes of shares available through other financial intermediaries. An investor transacting in a class of Fund shares without any front-end sales charge, contingent deferred sales charge (CDSC), or other asset-based fee for sales or distribution, such as a Rule 12b-1 fee, may be required to pay a commission to the financial intermediary for effecting such transactions. Each investor’s personal situation is different and you may wish to discuss with your financial intermediary the share classes the Fund offers, which share classes are available to you and which share class(es) is/are appropriate for you. In all instances, it is your responsibility to notify your financial intermediary or (for Direct-at-Fund Accounts, as

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

defined below) the Fund at the time of purchase of any relationship or other facts that may qualify you for sales charge waivers or discounts. The Fund, the Distributor and the Transfer Agent do not provide investment advice or make recommendations regarding Fund share classes. Your financial intermediary may provide advice and recommendations to you, such as which share class(es) is/are appropriate for you.

When deciding which class of shares to buy, you should consider, among other things:

■

The amount you plan to invest.

■

How long you intend to remain invested in the Fund.

■

The fees (e.g., sales charge or “load”) and expenses for each share class.

■

Whether you may be eligible for a reduction or waiver of sales charges when you buy or sell shares.

FUNDamentals

Front-End Sales Charge Calculation

The front-end sales charge is calculated as a percentage of the offering price.

■

The net asset value (NAV) per share is the price of a share calculated by the Fund every business day.

■

The offering price per share is the NAV per share plus any front-end sales charge (or load) that applies.

The dollar amount of any applicable front-end sales charge is the difference between the offering price of the shares you buy and the NAV of those shares. To determine the front-end sales charge you will pay when you buy Class A shares, the Fund will add the amount of your investment to the value of your account (and any other accounts eligible for aggregation of which you or your financial intermediary notifies the Fund) and base the sales charge on the aggregate amount. For information on account value aggregation, sales charge waivers and other important information, see Choosing a Share Class — Reductions/Waivers of Sales Charges.

FUNDamentals

Contingent Deferred Sales Charge

A contingent deferred sales charge (CDSC) is a sales charge applied at the time you sell your shares, unlike a front-end sales charge that is applied at the time of purchase. A CDSC can vary based on the length of time that you have held your shares. A CDSC is applied to the NAV at the time of your purchase or sale, whichever is lower, and will not be applied to any shares you receive through Fund distribution reinvestments or any amount that represents appreciation in the value of your shares. For purposes of calculating a CDSC, the start of the holding period is generally the first day of the month in which your purchase was made.

When you place an order to sell shares of a class that has a CDSC, the Fund will first redeem any shares that are not subject to a CDSC, followed by those you have held the longest. This means that if a CDSC is imposed, you cannot designate the individual shares being redeemed for U.S. federal income tax purposes. You should consult your tax advisor about the tax consequences of investing in the Fund. In certain circumstances, the CDSC may not apply. See Choosing a Share Class — Reductions/Waivers of Sales Charges for details.

Share Class Features

The following summarizes the primary features of Class A, Class Adv, Class C, Class Inst, Class Inst2, Class Inst3 and Class S shares.

Not all Funds offer every class of shares. The Fund offers the class(es) of shares set forth on the cover of this prospectus, as it may be supplemented from time to time, and may offer other share classes through a separate prospectus. Although certain share classes are generally closed to new and/or existing investors, information relating to these share classes is included in the table below because certain qualifying purchase orders are permitted, as described below.

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

The sales charge reductions and waivers available to investors who purchase and hold their Fund shares through different financial intermediaries may vary. Appendix A describes financial intermediary-specific reductions and/or waiver policies. A shareholder transacting in Fund shares through a financial intermediary identified in Appendix A should carefully read the terms and conditions of Appendix A. A reduction and/or waiver that is specific to a particular financial intermediary is not available to Direct-at-Fund Accounts, as defined below, or through another financial intermediary. The information in Appendix A may be provided by, or compiled from or based on information provided by the financial intermediaries identified in Appendix A. To obtain additional information regarding any sales charge reduction and/or waiver described in Appendix A, and to ensure that you receive any such reductions or waivers that may be available to you, please consult your financial intermediary.

Share Class	Eligible Investors; Minimum Initial Investments; Conversion Features(a)	Front-End Sales Charges(b)	Contingent Deferred Sales Charges (CDSCs)(b)	Sales Charge Reductions/Waivers	Maximum Distribution and/or Service (12b-1) Fees(c)
Class A	Eligibility: Available to the general public for investment Minimum Initial Investment: $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts (as described below))	5.75% maximum, declining to 0.00% on investments of $1 million or more	CDSC on certain investments of between $1 million and $50 million redeemed within 18 months after purchase charged as follows: • 1.00% CDSC if redeemed within 12 months after purchase and • 0.50% CDSC if redeemed more than 12, but less than 18, months after purchase	Reductions: yes, see Choosing a Share Class — Reductions/Waivers of Sales Charges – Class A Shares Front-End Sales Charge Reductions Waivers: yes, on Fund distribution reinvestments. For additional waivers, see Choosing a Share Class — Reductions/Waivers of Sales Charges – Class A Shares Front-End Sales Charge Waivers, as well as Choosing a Share Class — CDSC Waivers – Class A and Class C Financial intermediary-specific waivers are also available, see Appendix A	Service Fee: 0.25%
Class Adv	Eligibility: Available only to (i) omnibus retirement plans, including self-directed brokerage accounts within omnibus retirement plans that clear through institutional no transaction fee (NTF) platforms; retirement plans; (ii) trust companies or similar institutions; (iii) broker- dealers, banks, trust companies and similar institutions that clear Fund share transactions for their client or customer investment advisory or similar accounts through designated financial intermediaries and their mutual fund trading platforms that have been granted specific written authorization from the Transfer Agent with respect to Class Adv eligibility apart from selling, servicing or similar agreements; (iv) 501(c)(3) charitable organizations; (v) 529 plans; (vi) health savings accounts; (vii) investors participating in a fee- based advisory program sponsored by a financial intermediary or other entity that is not compensated by the Fund for those services, other than payments for shareholder servicing or sub-accounting performed in place	None	None	N/A	None

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Share Class	Eligible Investors; Minimum Initial Investments; Conversion Features(a)	Front-End Sales Charges(b)	Contingent Deferred Sales Charges (CDSCs)(b)	Sales Charge Reductions/Waivers	Maximum Distribution and/or Service (12b-1) Fees(c)
	of the Transfer Agent; and (viii) commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Adv shares within such platform. Minimum Initial Investment: None, except in the case of (viii) above, which is $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts)
Class C	Eligibility: Available to the general public for investment Minimum Initial Investment: $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts) Conversion Feature: Yes. Effective April 1, 2021, Class C shares generally automatically convert to Class A shares of the same Fund in the month of or the month following the 8-year anniversary of the Class C shares purchase date. Prior to April 1, 2021, Class C shares generally automatically converted to Class A shares of the same Fund in the month of or the month following the 10-year anniversary of the Class C shares purchase date.(a)	None	1.00% on certain investments redeemed within one year of purchase	Waivers: yes, on Fund distribution reinvestments. For additional waivers, see Choosing a Share Class — CDSC Waivers – Class A and Class C Financial intermediary-specific CDSC waivers are also available, see Appendix A	Distribution Fee: 0.75% Service Fee: 0.25%
Class Inst	Eligibility: Available only to certain eligible investors, which are subject to different minimum investment requirements, ranging from $0 to $2,000, including investors who purchase Fund shares through commissionable brokerage platforms where the financial intermediary holds the shares in an omnibus account and, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst shares within such platform; closed to (i) accounts of financial intermediaries that clear Fund share transactions for their client or customer accounts through designated financial intermediaries and their mutual fund	None	None	N/A	None

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Share Class	Eligible Investors; Minimum Initial Investments; Conversion Features(a)	Front-End Sales Charges(b)	Contingent Deferred Sales Charges (CDSCs)(b)	Sales Charge Reductions/Waivers	Maximum Distribution and/or Service (12b-1) Fees(c)
	trading platforms that have been given specific written notice from the Transfer Agent of the termination of their eligibility for new purchases of Class Inst shares and (ii) omnibus group retirement plans, subject to certain exceptions(d) Minimum Initial Investment: See Eligibility above
Class Inst2	Eligibility: Available only to (i) certain registered investment advisers and family offices that clear Fund share transactions for their client or customer accounts through designated financial intermediaries and their mutual fund trading platforms that have been granted specific written authorization from the Transfer Agent with respect to Class Inst2 eligibility apart from selling, servicing or similar agreements; (ii) omnibus retirement plans(d); (iii) health savings accounts, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst2 shares within such platform and that Fund shares are held in an omnibus account; and (iv) institutional investors that are clients of the Columbia Threadneedle Global Institutional Distribution Team that invest in Class Inst2 shares for their own account through platforms approved by the Distributor or an affiliate thereof to offer and/or service Class Inst2 shares within such platform. Minimum Initial Investment: None	None	None	N/A	None
Class Inst3	Eligibility: Available to: (i) group retirement plans that maintain plan- level or omnibus accounts with the Fund(d); (ii) institutional investors that are clients of the Columbia Threadneedle Global Institutional Distribution Team that invest in Class Inst3 shares for their own account through platforms approved by the Distributor or an affiliate thereof to offer and/or service Class Inst3 shares within such platform; (iii) collective trust funds; (iv) affiliated or unaffiliated mutual funds (e.g., funds operating as funds- of-funds); (v) fee-based platforms of financial intermediaries (or the clearing intermediary they trade through) that have an agreement with the	None	None	N/A	None

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Share Class	Eligible Investors; Minimum Initial Investments; Conversion Features(a)	Front-End Sales Charges(b)	Contingent Deferred Sales Charges (CDSCs)(b)	Sales Charge Reductions/Waivers	Maximum Distribution and/or Service (12b-1) Fees(c)
	Distributor or an affiliate thereof that specifically authorizes the financial intermediary to offer and/or service Class Inst3 shares within such platform, provided also that Fund shares are held in an omnibus account; (vi) commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst3 shares within such platform and that Fund shares are held in an omnibus account; (vii) health savings accounts, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst3 shares within such platform and that Fund shares are held in an omnibus account; and (viii) bank trust departments, subject to an agreement with the Distributor that specifically authorizes offering Class Inst3 shares and provided that Fund shares are held in an omnibus account. In each case above where noted that Fund shares are required to be held in an omnibus account, the Distributor, may, in its discretion, determine to waive this requirement. Minimum Initial Investment: No minimum for the eligible investors described in (i), (iii), (iv) and (v) above; $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts) for the eligible investors described in (vi) above; and $1 million for all other eligible investors, unless waived in the discretion of the Distributor
Class S	Effective on or about October 1, 2024, Class S shares are available (i) to be held, but not purchased, by accounts of financial intermediaries that, with specific written permission from the Transfer Agent, have instructed the Fund to exchange Class Inst shares for Class S shares; and (ii) to be held and purchased by omnibus group retirement plans that (A) opened and, subject to exceptions, funded a Class Inst share account as of the close of business on March 28, 2013 and have continuously held Class Inst	None	None	N/A	None

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Share Class	Eligible Investors; Minimum Initial Investments; Conversion Features(a)	Front-End Sales Charges(b)	Contingent Deferred Sales Charges (CDSCs)(b)	Sales Charge Reductions/Waivers	Maximum Distribution and/or Service (12b-1) Fees(c)
	shares in such account through at least September 30, 2024 and (B) received specific written permission from the Transfer Agent to exchange Class Inst shares for Class S shares. Only omnibus retirement plans referenced in (ii) above may purchase Class S shares, open new Class S accounts, or add new plan participants to a Class S shares omnibus retirement plan account. Otherwise, Class S shares are not available for purchase (other than through the reinvestment of distributions on Class S shares).(d) Minimum Initial Investment: See Eligibility above

(a)

For more information on the conversion of Class C shares to Class A shares, see Choosing a Share Class — Sales Charges and Commissions — Class C Shares —Conversion to Class A Shares.

(b)

For more information on applicable sales charges, see Choosing a Share Class — Sales Charges and Commissions, and for information about certain exceptions to these sales charges, see Choosing a Share Class — Reductions/Waivers of Sales Charges.

(c)

These are the maximum applicable distribution and/or service fees under the Fund’s Rule 12b-1 plan. Because these fees are paid out of Fund assets on an ongoing basis, over time these fees will increase the cost of your investment and may cost you more than paying other types of distribution and/or service fees. For more information on distribution and service fees, see Choosing a Share Class — Distribution and Service Fees.

(d)

These share classes are closed to new accounts generally or to new accounts of certain categories of investors, subject to certain conditions, as summarized below and described in more detail under Buying, Selling and Exchanging Shares — Buying Shares — Eligible Investors:

• Class Inst Shares: Financial intermediaries that clear Fund share transactions through designated financial intermediaries and their mutual fund trading platforms that were given specific written notice from the Transfer Agent of the termination, effective March 29, 2013, of their eligibility for new purchases of Class Inst shares and omnibus retirement plans are not permitted to establish new Class Inst accounts, subject to certain exceptions. Omnibus retirement plans that opened and, subject to exceptions, funded a Class Inst account as of close of business on March 28, 2013, and have continuously held Class Inst shares in such account after such date, may generally continue to make additional purchases of Class Inst shares, open new Class Inst accounts and add new participants. In certain circumstances and in the sole discretion of the Distributor, omnibus retirement plans affiliated with a grandfathered plan may also open new Class Inst accounts. Accounts of financial intermediaries (other than omnibus retirement plans) that clear Fund share transactions for their client or customer accounts through designated financial intermediaries and their mutual fund trading platforms are not permitted to establish new Class Inst accounts or make additional purchases of Class Inst shares (other than through reinvestment of distributions).

• Class Inst2 Shares. Shareholders with Class Inst2 accounts funded before November 8, 2012 who do not satisfy the current eligibility criteria for Class Inst2 shares may not establish new Class Inst2 accounts but may continue to make additional purchases of Class Inst2 shares in existing accounts. In addition, investment advisory programs and similar programs that opened a Class Inst2 account as of May 1, 2010, and continuously hold Class Inst2 shares in such account after such date, may generally not only continue to make additional purchases of Class Inst2 shares but also open new Class Inst2 accounts and add new shareholders in the program.

• Class Inst3 Shares: Shareholders with Class Inst3 accounts funded before November 8, 2012 who do not satisfy the current eligibility criteria for Class Inst3 shares may not establish new accounts for such share class but may continue to make additional purchases of Class Inst3 shares in existing accounts.

• Class S Shares. Effective on or about October 1, 2024, Class S shares are available (i) to be held, but not purchased, by accounts of financial intermediaries that, with specific written permission from the Transfer Agent, have instructed the Fund to exchange Class Inst shares for Class S shares; and (ii) to be held and purchased by omnibus group retirement plans that (A) opened and, subject to exceptions, funded a Class Inst share account as of the close of business on March 28, 2013 and have continuously held Class Inst shares in such account through at least September 30, 2024 and (B) received specific written permission from the Transfer Agent to exchange Class Inst shares for Class S shares. Only omnibus retirement plans referenced in (ii) above may purchase Class S shares, open new Class S accounts, or add new plan participants to a Class S shares omnibus retirement plan account. Otherwise, Class S shares are not available for purchase (other than through the reinvestment of distributions on Class S shares).

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Sales Charges and Commissions

Sales charges, commissions, and distribution fees compensate financial intermediaries (typically your financial advisor) for selling shares to you, and service fees compensate financial intermediaries for maintaining and servicing the shares held in your account with them. Distribution and service fees are discussed in a separate sub-section below. Depending on which share class you choose and the financial intermediary through which you purchase your shares, you may pay these charges at potentially different levels at the outset as a front-end sales charge, at the time you sell your shares as a CDSC and/or over time in the form of distribution and/or service fees. You may be required to pay brokerage commissions and other fees to financial intermediaries when transacting in any class of Fund shares, including those that do not assess any front-end sales charge, contingent deferred sales charge, or other asset-based fee for sales or distribution. Such brokerage commissions and other fees are set by the financial intermediary.

As described in more detail below, Class A shares have a front-end sales charge, which is deducted from your purchase price when you buy your shares, and results in a smaller dollar amount being invested in the Fund than the purchase price you pay (unless you qualify for a waiver or reduction of the sales charge). The Fund’s other share classes do not have a front-end sales charge, so the full amount of your purchase price is invested in those classes. Class A shares have lower ongoing distribution and/or service fees than Class C shares of the Fund, if available. Over time, Class C shares can incur distribution and/or service fees that are equal to or more than the front-end sales charge and the distribution and/or service fees you would pay for Class A shares. Although the full amount of your purchase price of Class C shares is invested in a Fund, your return on this money will be reduced by the expected higher annual expenses of Class C shares. In this regard, Class C shares held in Direct-at-Fund Accounts automatically convert to Class A shares of the same Fund in the month of or the month following the 8-year anniversary of the Class C shares purchase date. The Fund may convert Class C shares held through a financial intermediary to Class A shares sooner in connection with the withdrawal of Class C shares of the Fund from the financial intermediary’s platform or accounts. No sales charge or other charges will apply in connection with such conversions, and the conversions are free from U.S. federal income tax. Once your Class C shares convert to Class A shares, your total returns from an investment in the Fund may increase as a result of the lower operating costs of Class A shares. Class Adv, Class Inst, Class Inst2, Class Inst3 and Class S shares of the Fund do not have distribution and/or service fees.

Whether the ultimate cost is higher for one share class over another depends on the amount you invest, how long you hold your shares, the fees (i.e., sales charges) and expenses of the class and whether you are eligible for reduced or waived sales charges, if available. You are responsible for choosing the share class most appropriate for you after taking into account your share class eligibility, class-specific features, and any applicable reductions in, or waivers of, sales charges. For more information, see Choosing a Share Class – Reductions/Waivers of Sales Charges. We encourage you to consult with a financial advisor who can help you with your investment decisions. Please contact your financial intermediary for more information about services, fees and expenses, and other important information about investing in the Fund, as well as with any questions you may have about your investing options. In all instances, it is your responsibility to notify your financial intermediary or (for Direct-at-Fund Accounts, as defined below) the Fund at the time of purchase of any relationship or other facts that may qualify you for sales charge waivers or discounts.

Class A Shares — Front-End Sales Charge

Unless your purchase qualifies for a waiver (e.g., you buy the shares through reinvested Fund dividends or distributions or subject to an applicable financial intermediary-specific waiver), you will pay a front-end sales charge when you buy Class A shares, resulting in a smaller dollar amount being invested in a Fund than the purchase price you pay. The Class A shares sales charge is waived on Class C shares converted to Class A shares. For more information about sales charge waivers and reduction opportunities, see Choosing a Share Class — Reductions/Waivers of Sales Charges and Appendix A.

The Distributor receives the sales charge and re-allows (or pays) a portion of the sales charge to the financial intermediary through which you purchased the shares. The Distributor retains the balance of the sales charge.

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

The front-end sales charge you will pay on Class A shares:

■

depends on the amount you are investing (generally, the larger the investment, the smaller the percentage sales charge), and

■

is based on the total amount of your purchase and the value of your account (and any other accounts eligible for aggregation of which you or your financial intermediary notifies the Fund).

The table below presents the front-end sales charge as a percentage of both the offering price and the net amount invested.

Class A Shares — Front-End Sales Charge — Breakpoint Schedule
Breakpoint Schedule For:	Dollar amount of shares bought(a)	Sales charge as a % of the offering price(b)	Sales charge as a % of the net amount invested(b)	Amount retained by or paid to financial intermediaries as a % of the offering price
Class A Shares	$0–$49,999	5.75%	6.10%	5.00%
	$50,000–$99,999	4.50%	4.71%	3.75%
	$100,000–$249,999	3.50%	3.63%	3.00%
	$250,000–$499,999	2.50%	2.56%	2.15%
	$500,000–$999,999	2.00%	2.04%	1.75%
	$1,000,000 or more	0.00%	0.00%	0.00%(c)

(a)

Purchase amounts and account values may be aggregated among all eligible Fund accounts for the purposes of this table. See Choosing a Share Class — Reductions/Waivers of Sales Charges for a discussion of account value aggregation.

(b)

Because the offering price is calculated to two decimal places, the dollar amount of the sales charge as a percentage of the offering price and your net amount invested for any particular purchase of Fund shares may be higher or lower depending on whether downward or upward rounding was required during the calculation process. Purchase price includes the sales charge.

(c)

For information regarding cumulative commissions paid to your financial intermediary when you buy $1 million or more of Class A shares of a Fund, see Class A Shares — Commissions below.

Class A Shares — CDSC

In some cases, you'll pay a CDSC if you sell Class A shares that you purchased without a front-end sales charge.

■

If you purchased Class A shares without an initial sales charge because your accounts aggregated between $1 million and $50 million at the time of purchase, you will incur a CDSC if you redeem those shares within 18 months after purchase, which is charged as follows: 1.00% CDSC if shares are redeemed within 12 months after purchase; and 0.50% CDSC if shares are redeemed more than 12, but less than 18, months after purchase.

■

Subsequent Class A share purchases that bring your aggregate account value to $1 million or more (but less than $50 million) will also be subject to a CDSC if you redeem them within the time periods noted above.

Class A Shares — Commissions

The Distributor may pay your financial intermediary an up-front commission when you buy Class A shares. The Distributor generally funds the commission through the applicable sales charge you paid. For more information, see Class A Shares — Front-End Sales Charge above.

The Distributor may also pay your financial intermediary a cumulative commission when you buy Class A shares in amounts not subject to a front-end sales charge, according to the following schedules (assets initially purchased into Class A shares of Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration

Prospectus 2024

Columbia Acorn® Fund

Choosing a Share Class (continued)

Municipal Bond Fund, Columbia Ultra Short Term Bond Fund and Columbia U.S. Treasury Index Fund that were purchased without the application of a front-end sales charge are excluded for purposes of calculating a financial intermediary’s commission under these schedules):

Class A Shares — Commission Schedule (Paid by the Distributor to Financial Intermediaries)
Purchase Amount	Commission Level* (as a % of net asset value per share)
$1 million – $2,999,999	1.00%
$3 million – $49,999,999	0.50%
$50 million or more	0.25%

The commission level applies to the applicable asset level; therefore, for example, for a purchase of $5 million, the Distributor would pay a commission of 1.00% on the first $2,999,999 and 0.50% on the balance.

Class C Shares — Front-End Sales Charge

You do not pay a front-end sales charge when you buy Class C shares, but you may pay a CDSC when you sell Class C shares. Although Class C shares do not have a front-end sales charge, over time Class C shares can incur distribution and/or service fees that are equal to or more than the front-end sales charge and distribution and/or service fees you would pay for Class A shares. Thus, although the full amount of your purchase of Class C shares is invested in a Fund, any positive investment return on this money may be partially or fully offset by the expected higher annual expenses of Class C shares.

Class C Shares — Conversion to Class A Shares

Class C shares of a Fund generally automatically convert to Class A shares of the same Fund in the month of or the month following the 8-year anniversary of the Class C shares purchase date. Class C shares held through a financial intermediary in an omnibus account will be converted (pursuant to the financial intermediary’s Class C conversion policy, including those disclosed in Appendix A, which may differ from the Fund’s policy described here) provided that the intermediary is able to track individual shareholders’ holding periods. It is the financial intermediary's (and not the Fund's) responsibility to keep records and to ensure that the shareholder holding period is calculated properly. Not all financial intermediaries are able to track individual shareholders' holding periods. For example, group retirement plans held through third-party intermediaries that hold Class C shares in an omnibus account may not track participant level share lot aging. Please consult with your financial intermediary about your eligibility for Class C share conversion. The Fund may convert Class C shares held through a financial intermediary to Class A shares sooner in connection with the withdrawal of Class C shares of the Fund from the financial intermediary's platform or accounts. Once your Class C shares convert to Class A shares, your total returns from an investment in the Fund may increase as a result of the lower operating costs of Class A shares.

The following rules apply to the automatic conversion of Class C shares to Class A shares:

■

Class C share accounts that are Direct-at-Fund Accounts and Networked Accounts for which the Transfer Agent (and not your financial intermediary) sends you Fund account transaction confirmations and statements, convert on or about the 15th day of the month (if the 15th is not a business day, then the next business day thereafter) that they become eligible for automatic conversion provided that the Fund has records that Class C shares have been held for the requisite time period.

■

For purposes of determining the month when your Class C shares are eligible for conversion, the start of the holding period is the first day of the month in which your purchase was made. Your financial intermediary may choose a different day of the month to convert Class C shares. Please contact your financial intermediary for more information on calculating the holding period.

■

Any shares you received from reinvested distributions on these shares generally will convert to Class A shares at the same time.

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Columbia Acorn® Fund

Choosing a Share Class (continued)

■

You’ll receive the same dollar value of Class A shares as the Class C shares that were automatically converted. Class C shares that you received from an exchange of Class C shares of another Fund will convert based on the day you bought the original shares.

■

In addition to the above automatic conversion of Class C to Class A shares policy, the Transfer Agent seeks to convert Class C shares as soon as administratively feasible, regardless of how long such shares have been owned, to Class A shares of the same Fund for Direct-at-Fund Accounts (as defined below) that do not or no longer have a financial intermediary assigned to them. Direct-at-Fund Accounts that do not have a financial intermediary assigned to them are not permitted to purchase Class C shares; Class C share purchase orders received by Direct-at-Fund Accounts that do not have a financial intermediary assigned to the account will automatically be invested in Class A shares of the same Fund.

■

No sales charge or other charges apply in connection with these automatic conversions, and the conversions are free from U.S. federal income tax.

Class C Shares — CDSC

You will pay a CDSC of 1.00% if you redeem Class C shares within 12 months of buying them unless you qualify for a waiver of the CDSC (e.g., the shares you are selling were purchased with reinvested Fund distributions). Redemptions of Class C shares are not subject to a CDSC if redeemed after 12 months. For more information, see Choosing a Share Class — Reductions/Waivers of Sales Charges.

Class C Shares — Commissions

Although there is no front-end sales charge when you buy Class C shares, the Distributor makes an up-front payment (which includes a sales commission and an advance of service fees) directly to your financial intermediary of up to 1.00% of the NAV per share when you buy Class C shares. A portion of this payment may be passed along to your financial advisor. The Distributor seeks to recover this payment through distribution fees of up to 0.75% and shareholder service fees of up to 0.25% it receives under the Fund's Rule 12b-1 plan during the first 12 months following the sale of Class C shares, and any applicable CDSC when you sell your shares. For more information, see Choosing a Share Class — Distribution and Service Fees.

Reductions/Waivers of Sales Charges

The availability of certain sales charge waivers and discounts will depend on whether you purchase your shares directly from the Fund (i.e., a Direct-at-Fund Account, as defined below) or through a financial intermediary. Financial intermediaries may have different policies and procedures regarding the availability of front-end sales charge and/or CDSC waivers. In all instances, it is your responsibility to notify your financial intermediary or (for Direct-at-Fund Accounts, as defined below) the Fund at the time of purchase of any relationship or other facts that may qualify you for sales charge waivers or discounts. In order to obtain waivers and discounts not available through a particular financial intermediary, shareholders will have to purchase Fund shares directly from the Fund (if permitted) or through a different financial intermediary. For a description of financial intermediary-specific sales charge reductions and/or waivers, see Appendix A.

Class A Shares Front-End Sales Charge Reductions

The Fund makes available two means of reducing the front-end sales charge that you may pay when you buy Class A shares of a Fund. These types of sales charge reductions are also referred to as breakpoint discounts.

First, through the right of accumulation (ROA), you may combine the value of eligible accounts (as described in the Eligible Accounts section below) maintained by you and members of your immediate family to reach a breakpoint discount level and apply a lower front-end sales charge to your purchase. To calculate the combined value of your eligible Fund accounts in the particular class of shares, the Fund will use the current public offering price per share. For purposes of obtaining a breakpoint discount through ROA, you may aggregate your and your “immediate family” members' ownership (as described in the FUNDamentals box below) of certain classes of shares held in certain account types, as described in the Eligible Accounts section below.

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Columbia Acorn® Fund

Choosing a Share Class (continued)

Second, by making a statement of intent to purchase additional shares (commonly referred to as a letter of intent (LOI)), you may pay a lower sales charge on all purchases of Class A shares made within 13 months after the date of your LOI. Your LOI must state the aggregate amount of purchases you intend to make in that 13-month period, which must be at least enough to reach the first (or next) breakpoint of the Fund. The required form of LOI may vary by financial intermediary, so please contact them directly for more information. Five percent of the purchase commitment amount will be placed in escrow (which means you will not be able to redeem Fund shares equivalent to that amount until they are released from escrow). At the end of the 13-month period, the shares will be released from escrow, provided that you have invested the commitment amount. If you do not invest the commitment amount by the end of the 13 months, the remaining amount of the unpaid sales charge will be redeemed from the escrowed shares and the remaining balance released from escrow. To calculate the total value of the purchases you've made under an LOI, the Fund will use the historic cost (i.e., dollars invested and not current market value) of the shares held in each eligible account; reinvested dividends or capital gains, or purchases made through the reinstatement privilege do not count as purchases made under an LOI. For purposes of making an LOI to purchase additional shares, you may aggregate eligible shares owned by you or your immediate family members in eligible accounts, valued as of the day immediately before the initiation of your LOI.

You must request the reduced sales charge (whether through ROA or an LOI) when you buy shares. If you do not complete and file an LOI, or do not request the reduced sales charge at the time of purchase, you will not be eligible for the reduced sales charge. To obtain a breakpoint discount, you must notify your financial intermediary in writing at the time you buy your shares of each eligible account maintained by you and members of your immediate family, including accounts maintained through different financial intermediaries. You and your financial intermediary are responsible for ensuring that you receive discounts for which you are eligible. Please contact your financial intermediary with questions regarding application of the eligible discount to your account. You may be asked by your financial intermediary (or by the Fund if you hold your account directly with the Fund) for account statements or other records to verify your discount eligibility for new and subsequent purchases, including, when applicable, records for accounts opened with a different financial intermediary and records of accounts established by members of your immediate family.

The sales charge reductions available to investors who purchase and hold their Fund shares through different financial intermediaries may vary. For a description of such financial intermediary-specific sales charge reductions, see Appendix A.

FUNDamentals

Your “Immediate Family” and Account Value Aggregation

For purposes of obtaining a breakpoint discount for Class A shares, the value of your account will be deemed to include the value of all applicable shares in eligible Fund accounts that are held by you and your “immediate family,” which includes your spouse, domestic partner, parent, step-parent, legal guardian, child under 21, step-child under 21, father-in-law and mother-in-law, provided that you and your immediate family members share the same mailing address. Any Fund accounts linked together for account value aggregation purposes as of the close of business on September 3, 2010 will be permitted to remain linked together. Group retirement plan accounts are valued at the retirement plan level.

Eligible Accounts

The following accounts are eligible for account value aggregation as described above, provided that they are invested in Class A (excluding, in the case of Direct-at-Fund Accounts, Funds that do not assess a front-end sales charge, including Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration Municipal Bond Fund, Columbia Ultra Short Term Bond Fund and Columbia U.S. Treasury Index Fund, unless such shares were purchased via an exchange from Class A shares of a Fund on which you paid the Class A share applicable front-end sales charge), Class C, Class E or Class Inst shares of a Fund, or non-retirement plan accounts invested in Class Adv, Class Inst2, Class Inst3 or Class S shares of a Fund: individual or joint accounts; Roth and traditional

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Columbia Acorn® Fund

Choosing a Share Class (continued)

Individual Retirement Accounts (IRAs); Simplified Employee Pension accounts (SEPs), Savings Investment Match Plans for Employees of Small Employers accounts (SIMPLEs) and Tax Sheltered Custodial Accounts (TSCAs); Uniform Gifts to Minors Act (UGMA)/Uniform Transfers to Minors Act (UTMA) accounts for which you, your spouse, or your domestic partner is parent or guardian of the minor child; revocable trust accounts for which you or an immediate family member, individually, is the beneficial owner/grantor; accounts held in the name of your, your spouse’s, or your domestic partner’s sole proprietorship or single owner limited liability company or S corporation; qualified retirement plan assets, provided that you are the sole owner of the business sponsoring the plan, are the sole participant (other than a spouse) in the plan, and have no intention of adding participants to the plan; and investments in wrap accounts.

The following accounts are not eligible for account value aggregation as described above: accounts of pension and retirement plans with multiple participants, such as 401(k) plans (which are combined to reduce the sales charge for the entire pension or retirement plan and therefore are not used to reduce the sales charge for your individual accounts); investments in 529 plans, donor advised funds, variable annuities, variable insurance products or managed separate accounts; charitable and irrevocable trust accounts; accounts holding shares of money market funds that used the Columbia brand before May 1, 2010; accounts invested in Class R shares of a Fund; and retirement plan accounts invested in Class Adv, Class Inst2, Class Inst3 or Class S shares of a Fund.

Additionally, direct purchases of shares of Columbia Government Money Market Fund may not be aggregated for account value aggregation purposes; however, shares of Columbia Government Money Market Fund acquired by exchange from other Columbia Funds that assess a sales charge may be included in account value aggregation.

Class A Shares Front-End Sales Charge Waivers

There are no front-end sales charges on reinvested Fund distributions. The Class A shares sales charge is waived on conversions of Class C shares to Class A shares. The Distributor may waive front-end sales charges on purchases of Class A shares of the Funds by certain categories of investors, including Board members, certain employees of financial intermediaries, Fund portfolio managers, certain partners and employees of outside legal counsel to the Funds or the Board, separate accounts of an insurance company exempt from registration as an investment company under Section 3(c)(11) of the 1940 Act, registered broker-dealer firms that have an agreement with the Distributor purchasing Fund shares for their investment account only, and qualified employee benefit plan rollovers to Class A shares in the same Fund (see Appendix S to the SAI for details). For a more complete description of categories of investors who may purchase Class A shares of the Funds at NAV, without payment of any front-end sales charge that would otherwise apply, see Appendix S to the SAI.

In addition, certain types of purchases of Class A shares may be made at NAV. The Distributor may waive front-end sales charges on (i) purchases (including exchanges) of Class A shares in accounts of financial intermediaries that have entered into agreements with the Distributor to offer Fund shares to self-directed investment brokerage accounts that may or may not charge a transaction fee to customers; (ii) exchanges of Class Inst shares of a Fund for Class A shares of the Fund; (iii) purchases of Class A shares on brokerage mutual fund-only platforms of financial intermediaries that have an agreement with the Distributor that specifically authorizes the offering of Class A shares within such platform; (iv) purchases through certain wrap fee or other products or programs that involve fee-based compensation arrangements that have, or clear trades through a financial intermediary that has, a selling agreement with the Distributor; (v) purchases through state sponsored 529 Plans; (vi) purchases through banks, trust companies, and thrift institutions acting as fiduciaries; (vii) purchases through certain employee benefit plans and certain qualified deferred compensation plans; and (viii) purchases of Class A shares in Direct-at-Fund Accounts (as defined below) that do not have a financial intermediary assigned to them. For a more complete description of these eligible transactions, see Appendix S to the SAI.

The sales charge waivers available to investors who purchase and hold their Fund shares through different financial intermediaries may vary. For a description of such financial intermediary-specific sales charge waivers, see Appendix A.

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Columbia Acorn® Fund

Choosing a Share Class (continued)

CDSC Waivers – Class A and Class C

You may be able to avoid an otherwise applicable CDSC when you sell Class A or Class C shares of the Fund. This could happen because of the way in which you originally invested in the Fund, because of your relationship with the Funds or for other reasons. For example, the CDSC will be waived on redemptions of shares: in the event of the shareholder's death; for which no sales commission or transaction fee was paid to an authorized financial intermediary at the time of purchase; purchased through reinvestment of dividends and capital gain distributions; that result from required minimum distributions taken from retirement accounts due to the shareholder reaching the qualified age based on applicable IRS regulations; that result from returns of excess contributions made to retirement plans or individual retirement accounts (subject to certain conditions); initially purchased by an employee benefit plan (for Class A shares) and that are not connected with a plan level termination (for Class C shares); in connection with the Fund's Small Account Policy (which is described in Buying, Selling and Exchanging Shares — Transaction Rules and Policies); held within Direct-at-Fund Accounts that do not have a financial intermediary assigned to them; and by certain other investors and in certain other types of transactions or situations. Restrictions may apply to certain accounts and certain transactions. The Distributor may, in its sole discretion, authorize the waiver of the CDSC for additional classes of investors. The Fund may change or cancel these terms at any time. Any change or cancellation applies only to future purchases. For a more complete description of the available waivers of the CDSC on redemptions of Class A or Class C shares, see Appendix S to the SAI.

Repurchases (Reinstatements)

As noted in the table below, you can redeem shares of certain classes (see Redeemed Share Class below) and use such redemption proceeds to buy shares of the Corresponding Repurchase Class without paying an otherwise applicable sales charge and/or CDSC (other than, in the case of Direct-at-Fund Accounts, redemptions from Funds that do not assess a front-end sales charge, including Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration Municipal Bond Fund, Columbia Ultra Short Term Bond Fund and Columbia U.S. Treasury Index Fund, unless such shares were purchased via an exchange from Class A shares of a Fund on which you paid the Class A share applicable front-end sales charge) within 90 days, up to the amount of the redemption proceeds.

Repurchases (Reinstatements)
Redeemed Share Class	Corresponding Repurchase Class
Class A	Class A
Class C	Class C

Any CDSC paid upon redemption of your Class A or Class C shares of a Fund will not be reimbursed.

To be eligible for the repurchase (or reinstatement) privilege, the purchase must be made into an account for the same owner, but does not need to be into the same Fund from which the shares were sold. The Transfer Agent, Distributor or their agents must receive a written reinstatement request from you or your financial intermediary within 90 days after the shares are redeemed. The purchase of the Corresponding Repurchase Class (as noted in the table above) through this repurchase (or reinstatement) privilege will be made at the NAV of such shares next calculated after the request is received in “good form.” Systematic withdrawals and purchases are excluded from this policy.

Restrictions and Changes in Terms and Conditions

Restrictions may apply to certain accounts and certain transactions. The Funds and/or the Distributor may change or cancel these terms and conditions at any time. Unless you provide your financial intermediary with information in writing about all of the factors that may count toward available reductions or waivers of an applicable sales charge, there can be no assurance that you will receive all of the reductions and waivers for which you may be eligible. To the

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Columbia Acorn® Fund

Choosing a Share Class (continued)

extent your Fund account is held directly with the Fund, you should provide this information to the Fund when placing your purchase or redemption order. Please see Appendix A to this prospectus and Appendix S of the SAI for more information about sales charge waivers.

Distribution and Service Fees

The Board has approved, and the Columbia Acorn Funds have adopted, a distribution plan pursuant to Rule 12b-1 under the 1940 Act that sets the distribution and/or service fees that are periodically deducted from the Funds’ assets. These fees are calculated daily, may vary by share class and are intended to compensate the Distributor and/or eligible financial intermediaries for, with regard to distribution fees, selling Fund shares and, with regard to service fees, directly or indirectly providing services to shareholders. Because the fees are paid out of the Fund's assets on an ongoing basis, they will increase the cost of your investment over time.

The table below shows the maximum annual distribution and/or service fees (as an annual percentage of average daily net assets) and the combined amount of such fees applicable to each share class of the Columbia Acorn Funds:

	Distribution Fee	Service Fee	Combined Total
Class A	None	0.25%	0.25%
Class Adv	None	None	None
Class C	0.75%	0.25%	1.00%
Class Inst	None	None	None
Class Inst2	None	None	None
Class Inst3	None	None	None
Class S	None	None	None

The distribution and/or service fees for Class A and Class C shares, as applicable, are subject to the requirements of Rule 12b-1 under the 1940 Act. The Distributor may retain these fees otherwise payable to financial intermediaries if the amounts due are below an amount determined by the Distributor in its sole discretion.

For Class A shares, the Distributor begins to pay these fees immediately after purchase, except in the following case, in which the Distributor begins to pay these fees 12 months after purchase: a purchase of Class A shares of $1 million or more that pay a Class A up-front commission to your financial intermediary and the financial intermediary has opted to receive such commission. The Distributor’s policy to otherwise begin to pay these fees immediately on Class A shares also applies to purchases of funds that do not pay an up-front sales commission on Class A shares, which includes Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration Municipal Bond Fund, Columbia Ultra Short Term Bond Fund and Columbia U.S. Treasury Index Fund. For Class C shares, the Distributor begins to pay these fees 12 months after purchase. However, for Class C shares, financial intermediaries may opt to decline the up-front payment described in Choosing a Share Class – Sales Charges and Commissions – Class C Shares – Commissions and instead may receive these fees immediately after purchase. If the intermediary opts to receive the up-front payment, the Distributor retains the distribution and service fee for the first 12 months following the sale of Class C shares in order to recover the up-front payment made to financial intermediaries and to pay for other related expenses.

If you maintain shares of the Fund directly with the Fund, without working with a financial advisor or other financial intermediary, distribution and service fees may be retained by the Distributor as payment or reimbursement for incurring certain distribution and shareholder service related expenses.

Over time, these distribution and/or service fees will reduce the return on your investment and may cost you more than paying other types of sales charges. The Fund will pay these fees to the Distributor and/or to eligible financial intermediaries for as long as the distribution plan continues in effect, which is expected to be indefinitely. However, the Fund may reduce or discontinue payments at any time. Your financial intermediary may also charge you other additional fees for providing services to your account, which may be different from those described here.

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Columbia Acorn® Fund

Choosing a Share Class (continued)

Financial Intermediary Compensation

The Distributor, the Investment Manager and their affiliates make payments, from their own resources, to financial intermediaries, including other Ameriprise Financial affiliates, for marketing/sales support services relating to the Funds (Marketing Support Payments). Such payments are generally based upon one or more of the following factors: average net assets of the Funds attributable to that financial intermediary; gross sales of the Funds attributable to that financial intermediary; reimbursement of ticket charges (fees that a financial intermediary charges its representatives for effecting transactions in Fund shares); or a negotiated lump sum payment. While the financial arrangements may vary for each financial intermediary, Marketing Support Payments to any one financial intermediary are generally between 0.01% and 0.40% on an annual basis for payments based on average net assets of the Fund attributable to the financial intermediary, and between 0.05% and 0.25% on an annual basis for firms receiving a payment based on gross sales of the Funds attributable to the financial intermediary. The Distributor, the Investment Manager and their affiliates make payments with respect to a Fund or the Columbia Funds generally on a basis other than those described above or in larger amounts when dealing with certain financial intermediaries. Not all financial intermediaries receive Marketing Support Payments. The Distributor, the Investment Manager and their affiliates do not make Marketing Support Payments with respect to Class Inst3 shares of the Columbia Acorn Funds.

In addition, the Transfer Agent has certain arrangements in place to compensate financial intermediaries, including other Ameriprise Financial affiliates, that hold Fund shares through networked and omnibus accounts, including omnibus retirement plans, for services that they provide to beneficial Fund shareholders (Shareholder Services). Shareholder Services and related fees vary by financial intermediary and according to distribution channel and may include sub-accounting, sub-transfer agency, participant recordkeeping, shareholder or participant reporting, shareholder or participant transaction processing, maintenance of shareholder records, preparation of account statements and provision of customer service, and are not intended to include services that are primarily intended to result in the sale of Fund shares. Payments for Shareholder Services vary by financial intermediary and according to distribution channel, but generally are not expected, with certain limited exceptions, to exceed 0.40% of the average aggregate value of the Fund’s shares. Payments to financial intermediaries in excess of the amounts paid by the Fund are borne by the Distributor, the Investment Manager and/or their affiliates.

Generally, each of the Columbia Acorn Funds compensates the Transfer Agent for payments made for Shareholder Services, for each share class except Class Inst3. Effective July 1, 2017, the Columbia Acorn Funds compensate the Transfer Agent for payments made for Shareholder Services as follows:

Financial Intermediary Distribution Channel	Fee Rate
Retirement	up to 0.18% of Fund assets held by intermediaries or platforms charging an asset-based fee, or $18 per account held by intermediaries or platforms charging a per account fee
Fund supermarket transaction fee (TF)	up to 0.12% of Fund assets held by intermediaries or platforms charging an asset-based fee, or $16 per account held for intermediaries or platforms charging a per account fee
Fund supermarket no-transaction fee (NTF)	up to 0.18% of Fund assets held by intermediaries or platforms charging an asset-based fee, or $18 per account held by intermediaries or platforms charging a per account fee
Bank	up to 0.10% of Fund assets held by intermediaries or platforms charging an asset-based fee, or $20 per account held by intermediaries or platforms charging a per account fee
Broker-dealer	up to 0.13% of Fund assets held by intermediaries or platforms charging an asset-based fee, or $16 per account held by intermediaries or platforms charging a per account fee

With respect to Class Inst2 and Class Inst3 shares of the Fund, total annual compensation payable to the Transfer Agent may not exceed, respectively, 0.07% and 0.02% of the average daily net assets of the Class. For additional details about the application of the Columbia Acorn Funds’ fee limits, please refer to the SAI.

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Columbia Acorn® Fund

Choosing a Share Class (continued)

The Transfer Agent does not pay financial intermediaries for Shareholder Services, and the Fund does not compensate the Transfer Agent for any Shareholder Services provided by financial intermediaries with respect to Class Inst3 shares of the Columbia Acorn Funds. The Fund believes that the fee levels paid to the Transfer Agent are reasonably related to the Shareholder Services provided, but payments may vary and there can be no assurance that the Fund will be able to identify with specificity the costs or particular Shareholder Services for which compensation is paid.

In addition to the payments described above, the Distributor, the Investment Manager and their affiliates typically make other payments or allow promotional incentives to certain broker-dealers to the extent permitted by the Securities and Exchange Commission (the SEC) and Financial Industry Regulatory Authority (FINRA) rules and by other applicable laws and regulations.

Amounts paid by the Distributor, the Investment Manager and their affiliates are paid out of their own resources and do not increase the amount paid by you or the Fund. You can find further details in the SAI about the payments made by the Distributor, the Investment Manager and their affiliates, as well as a list of the financial intermediaries, including Ameriprise Financial affiliates, to which the Distributor and the Investment Manager have agreed to make Marketing Support Payments and pay Shareholder Services fees.

Your financial intermediary may charge you fees and commissions in addition to those described in this prospectus. You should consult with your financial intermediary and review carefully any disclosure your financial intermediary provides regarding its services and compensation. Depending on the financial arrangement in place at any particular time, a financial intermediary and its financial advisors may have a conflict of interest or financial incentive for recommending the Fund or a particular share class over others.

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares

Share Price Determination

The price you pay or receive when you buy, sell or exchange shares is the Fund's next determined net asset value (or NAV) per share for a given share class. The Fund calculates the NAV per share for each class of shares of the Fund at the end of each business day, with the value of the Fund's shares based on the total value of all of the securities and other assets that it holds as of such specified time.

FUNDamentals

NAV Calculation

Each of the Fund's share classes calculates its NAV per share as follows:

NAV per share = (Value of assets of the share class) – (Liabilities of the share class)
Number of outstanding shares of the class

FUNDamentals

Business Days

A business day is any day that the New York Stock Exchange (NYSE) is open. A business day typically ends at the close of regular trading on the NYSE, usually at 4:00 p.m. Eastern time. If the NYSE is scheduled to close early, the business day will be considered to end as of the time of the NYSE’s scheduled close. The Fund will not treat an intraday unscheduled disruption in NYSE trading or an intraday unscheduled closing as a close of regular trading on the NYSE for these purposes and will price its shares as of the regularly scheduled closing time for that day (typically, 4:00 p.m. Eastern time). Nonetheless, the NAV of Fund shares may be determined at such other time or times (in addition to or in lieu of the time set forth above) as the Fund’s Board may approve or ratify. On holidays and other days when the NYSE is closed, the Fund’s NAV is not calculated and the Fund does not accept buy or sell orders. However, the value of the Fund’s assets may still be affected on such days to the extent that the Fund holds foreign securities that trade on days that foreign securities markets are open.

Equity securities listed on an exchange are typically valued at the closing price or last trade on their primary exchange at the close of business of the NYSE. Equity securities without a readily available closing price or that are not listed on any exchange are typically valued at the mean between the closing bid and asked prices. Other equity securities, debt securities and other assets are valued differently. For instance, bank loans trading in the secondary market are fair valued unless market quotations are readily available, fixed income investments maturing in 60 days or less are valued primarily using the amortized cost method, unless this methodology results in a valuation that does not approximate the market value of these securities, and those maturing in excess of 60 days are valued based on prices obtained from a pricing service, if available (which may represent market values or fair values). Investments in other open-end funds are valued at their published NAVs. The value of the Fund’s portfolio securities is determined in accordance with the valuation policy approved by the Board (the Valuation Policy). Pursuant to Rule 2a-5 under the 1940 Act, the Board has designated the Investment Manager as the Fund’s valuation designee. The Investment Manager, in turn, has authorized its valuation committee to make fair value determinations and to carry out supervisory and certain other functions relating to the valuation of Fund portfolio securities, pursuant to the Valuation Policy.

If a market price is not readily available or is deemed not to reflect market value for a portfolio security, the Investment Manager will determine the price based on a determination of the security's fair value pursuant to the Valuation Policy. In addition, the Investment Manager may use fair valuation techniques to price securities that trade on a foreign exchange when a significant event has occurred after the foreign exchange closes but before the time at which the Fund’s share price is calculated. Foreign exchanges typically close before the time at which Fund share prices are calculated, and may be closed altogether on days when the Fund is open. Such significant events affecting a foreign security may include, but are not limited to: (1) corporate actions, earnings announcements, litigation or other events impacting a single issuer; (2) governmental action that affects securities in one sector or country; (3)

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Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

natural disasters or armed conflicts affecting a country or region; or (4) significant domestic or foreign market fluctuations. The Fund uses various criteria, including an evaluation of U.S. market moves after the close of foreign markets, in determining whether a foreign security's market price is readily available and reflective of market value and, if not, the fair value of the security.

Fair valuation may have the effect of reducing stale pricing arbitrage opportunities presented by the pricing of Fund shares. However, when the Investment Manager uses fair valuation to price equity securities, it may value those securities higher or lower than another fund or manager would. Also, fair valuation of Fund holdings may cause the Fund's performance to diverge to a greater degree from the performance of various benchmark indices used to compare the Fund's performance because indices generally do not use fair valuation techniques. Because of the judgment involved in fair valuation decisions, there can be no assurance that the value ascribed to a particular security is accurate. The Fund has retained one or more independent fair valuation pricing services to assist in the fair valuation process for foreign securities.

Transaction Rules and Policies

The Fund, the Distributor or the Transfer Agent may refuse any order to buy or exchange shares. If this happens, the Fund will return any money it received, but no interest will be paid on that money. Your financial intermediary may have rules and policies in place that are in addition to or different than those described below.

Order Processing

Orders to buy, sell or exchange Fund shares are processed on business days. Depending upon the class of shares, orders can be made by mail, by telephone or online. Orders received in “good form” by the Transfer Agent or your financial intermediary before the end of a business day are priced at the NAV per share (plus any applicable sales charge) of the Fund's applicable share class on that day. Orders received after the end of a business day will receive the next business day's NAV per share (plus any applicable sales charge). For Direct-at-Fund Accounts (as defined below), when a written order to buy, sell or exchange shares is sent to the Transfer Agent, the share price used to fill the order is the next price calculated by the Fund after the Transfer Agent receives the transaction request in “good form” at its transaction processing center (i.e., the Fund’s express mail address), not the P.O. Box provided for regular mail delivery. The market value of the Fund's investments may change between the time you submit your order and the time the Fund next calculates its NAV per share. The business day that applies to your order is also called the trade date.

“Good Form”

An order is in “good form” if the Transfer Agent or your financial intermediary has received payment (in the case of purchases) and all of the information and documentation it deems necessary to effect your order. For example, when you sell shares, “good form” means that your request (i) has complete instructions and written requests include the signatures of all account owners, (ii) is for an amount that is less than or equal to the shares in your account for which payment has been received and collected, (iii) has a Medallion Signature Guarantee for amounts greater than $100,000 and certain other transactions, as described below, and (iv) includes any other required documents completed and attached. For the documents required for sales by corporations, agents, fiduciaries, surviving joint owners and other legal entities, call 800.345.6611.

Medallion Signature Guarantees

The Transfer Agent may require a Medallion Signature Guarantee for your signature in order to process certain transactions, including if: (i) the transaction amount is over $100,000; (ii) you want your check made payable to someone other than the registered account owner(s); (iii) the address of record has changed within the last 30 days; (iv) you want the check mailed to an address other than the address of record; (v) you want proceeds to be sent according to existing bank account instructions not coded for outgoing Automated Clearing House (ACH) or wire, or to a bank account not on file; or (vi) you are changing legal ownership of your account.

A Medallion Signature Guarantee helps assure that a signature is genuine and not a forgery. A Medallion Signature Guarantee must be provided by an eligible guarantor institution including, but not limited to, the following: a bank, credit union, savings association, broker or dealer that participates in the Securities Transfer Association Medallion

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Buying, Selling and Exchanging Shares (continued)

Program (STAMP), the Stock Exchange Medallion Program (SEMP) or the New York Stock Exchange Medallion Signature Program (MSP). For other transactions, the Transfer Agent may require a signature guarantee. Notarization by a notary public is not an acceptable signature guarantee. The Transfer Agent reserves the right to reject a signature guarantee and to request additional documentation for any transaction.

Customer Identification Program

Federal law requires the Fund to obtain and record specific personal information to verify your identity when you open an account. This information may include your name, address, date of birth (for individuals) and taxpayer or other government issued identification (e.g., social security number (SSN) or other taxpayer identification number (TIN)). If you fail to provide the requested information, the Fund may need to delay the date of your purchase or may be unable to open your account, which may result in a return of your investment monies. In addition, if the Fund is unable to verify your identity after your account is open, the Fund reserves the right to close your account or take other steps as deemed reasonable. The Fund will not be liable for any loss resulting from any purchase delay, application rejection or account closure due to a failure to provide proper identifying information.

Small Account Policy — Class A, Class C, Class Inst and Class S Share Accounts Below the Minimum Account Balance

The Funds generally will automatically sell your shares if the value of your Fund account (treating each account of the Fund you own separately from any other account of the Fund you may own) falls below the applicable minimum account balance. Any otherwise applicable CDSC will not be imposed on such an automatic sale of your shares. Generally, you may avoid such an automatic sale by raising your account balance to at least $250 or consolidating your multiple accounts you may have with the Funds through an exchange (so as to maintain at least $250 in each of your accounts). The minimum account balance varies among share classes and types of accounts, as follows:

Minimum Account Balance
	Minimum Account Balance
For all classes and account types except those listed below	$250 (None for accounts with Systematic Investment Plans)
Individual Retirement Accounts for all classes except those listed below	None
Class Adv, Class Inst2 and Class Inst3	None

For shares held directly with the Funds’ Transfer Agent, if your shares are sold, the Transfer Agent will remit the sale proceeds to you. The Transfer Agent will send you written notification in advance of any automatic sale, which will provide details on how you may avoid such an automatic sale. Generally, you may avoid such an automatic sale by raising your account balance to at least $250, consolidating your multiple accounts you may have with the Funds through an exchange (so as to maintain at least $250 in each of your accounts), or setting up a Systematic Investment Plan. For more information, contact the Transfer Agent or your financial intermediary. The Transfer Agent's contact information (toll-free number and mailing addresses) as well as the Funds’ website address can be found at the beginning of the section Choosing a Share Class.

For shares purchased and held for your benefit through a financial intermediary, the Funds may instruct the intermediary to automatically sell your Fund shares if the transaction can be operationally administered by the intermediary.

Small Account Policy — Class A, Class C, Class Inst and Class S Share Accounts Minimum Balance Fee

If the value of your Fund account (treating each account of the Fund you own separately from any other account of the Fund you may own) falls below the minimum initial investment requirement applicable to you for any reason, including as a result of market decline, your account generally could be subject to a $20 annual fee. The Transfer Agent will reduce the expenses paid by the Fund by any amounts it collects from the assessment of this fee. For Funds that do not have transfer agency expenses against which to offset the amount collected through assessment of this fee, the

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fee will be paid directly to the Fund. The Funds reserve the right to lower the account size trigger point for the minimum balance fee in any year or for any class of shares when we believe it is appropriate to do so in light of declines in the market value of Fund shares or for other reasons.

For shares held directly with the Funds’ Transfer Agent, this fee will be assessed through the automatic sale of Fund shares in your account. Any otherwise applicable CDSC will not be imposed on such an automatic sale of your shares. The Transfer Agent will send you written notification in advance of assessing any fee, which will provide details on how you can avoid the imposition of such fee. Generally, you may avoid the imposition of such fee by raising your Fund account balance, consolidating your multiple accounts you may have with the Funds, or setting up a Systematic Investment Plan that invests at least monthly. For more information, contact the Transfer Agent or your financial intermediary. The Transfer Agent's contact information (toll-free number and mailing addresses) as well as the Funds’ website address can be found at the beginning of the section Choosing a Share Class.

For shares purchased and held for your benefit through a financial intermediary, this fee could be assessed through the automatic sale of Fund shares in your account if instructed by the Fund and the transaction can be operationally administered by the intermediary.

Exceptions to the Small Account Policy (Accounts Below Minimum Account Balance) and Minimum Balance Fee

The automatic sale of Fund shares in accounts under $250 and the annual minimum balance fee described above do not apply to shareholders of Class Adv, Class Inst2 and Class Inst3 shares; shareholders holding their shares through financial intermediary networked accounts or trust networked accounts; wrap fee and omnibus accounts; accounts with active monthly Systematic Investment Plans; certain qualified retirement plans; and health savings accounts. The automatic sale of Fund shares of accounts under the applicable minimum account balance does not apply to individual retirement plans.

Small Account Policy — Financial Intermediary Networked and Wrap Fee Accounts

The Funds may automatically redeem, at any time, financial intermediary networked accounts and wrap fee accounts that have account balances of $20 or less or have less than one share.

Information Sharing Agreements

As required by Rule 22c-2 under the 1940 Act, the Funds or certain of their service providers will enter into information sharing agreements with financial intermediaries, including participating life insurance companies and financial intermediaries that sponsor or offer retirement plans through which shares of the Funds are made available for purchase. Pursuant to Rule 22c-2, financial intermediaries are required, upon request, to: (i) provide shareholder account and transaction information; and (ii) execute instructions from the Fund to restrict or prohibit further purchases of Fund shares by shareholders who have been identified by the Fund as having engaged in transactions that violate the Fund's excessive trading policies and procedures.

Excessive Trading Practices Policy

Right to Reject or Restrict Share Transaction Orders — The Fund is intended for investors with long-term investment purposes and is not intended as a vehicle for frequent trading activity (market timing) that is excessive. Investors should transact in Fund shares primarily for investment purposes. The Board has adopted excessive trading policies and procedures that are designed to deter excessive trading by investors (the Excessive Trading Policies and Procedures). The Fund discourages and does not accommodate excessive trading.

The Fund reserves the right to reject, without any prior notice, any purchase or exchange order for any reason, and will not be liable for any loss resulting from rejected orders. For example, the Fund may in its sole discretion restrict or reject a purchase or exchange order even if the transaction is not subject to the specific limitation described below if the Fund or its agents determine that accepting the order could interfere with efficient management of the Fund's

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Buying, Selling and Exchanging Shares (continued)

portfolio or is otherwise contrary to the Fund's best interests. The Excessive Trading Policies and Procedures apply equally to purchase or exchange transactions communicated directly to the Transfer Agent and to those received by financial intermediaries.

Specific Buying and Exchanging Limitations — If the Fund detects that an investor has made two “material round trips” in any 28-day period, it will generally reject the investor's future purchase orders, including exchange purchase orders, involving any Fund.

For these purposes, a “round trip” is a purchase or exchange into the Fund followed by a sale or exchange out of the Fund, or a sale or exchange out of the Fund followed by a purchase or exchange into the Fund. A “material” round trip is one that is deemed by the Fund to be material in terms of its amount or its potential detrimental impact on the Fund. Independent of this limit, the Fund may, in its sole discretion, reject future purchase orders by any person, group or account that appears to have engaged in any type of excessive trading activity.

These limits generally do not apply to automated transactions or transactions by registered investment companies in a “fund-of-funds” structure. These limits do not apply to payroll deduction contributions by retirement plan participants, transactions initiated by a retirement plan sponsor or certain other retirement plan transactions consisting of rollover transactions, loan repayments and disbursements, and required minimum distribution redemptions. They may be modified or rescinded for accounts held by certain retirement plans to conform to plan limits, for considerations relating to the Employee Retirement Income Security Act of 1974 or regulations of the Department of Labor, and for certain asset allocation or wrap programs. Accounts known to be under common ownership or control generally will be counted together, but accounts maintained or managed by a common intermediary generally will not be considered to be under common ownership or control. The Fund reserves the right to modify these restrictions at any time without prior notice to shareholders. In addition, the Fund may, in its sole discretion, reinstate trading privileges that have been revoked under the Fund's Excessive Trading Policies and Procedures.

Limitations on the Ability to Detect and Prevent Excessive Trading Practices — The Fund takes various steps designed to detect and prevent excessive trading, including daily review of available shareholder transaction information. However, the Fund receives buy, sell or exchange orders through financial intermediaries, and cannot always know of or reasonably detect excessive trading that may be facilitated by financial intermediaries or by the use of the omnibus account arrangements they offer. Omnibus account arrangements are common forms of holding shares of mutual funds, particularly among certain financial intermediaries such as broker-dealers, retirement plans and variable insurance products. These arrangements often permit financial intermediaries to aggregate their clients' transactions and accounts, and in these circumstances, the identities of the financial intermediary clients that beneficially own Fund shares are often not known to the Fund.

Some financial intermediaries apply their own restrictions or policies to their clients’ transactions and accounts, which may be more or less restrictive than those described here. This may impact the Fund's ability to curtail excessive trading, even where it is identified. For these and other reasons, it is possible that excessive trading may occur despite the Fund's efforts to detect and prevent it.

Although these restrictions and policies involve judgments that are inherently subjective and may involve some selectivity in their application, the Fund seeks to act in a manner that it believes is consistent with the best interests of Fund shareholders in making any such judgments.

Risks of Excessive Trading — Excessive trading creates certain risks to the Fund's long-term shareholders and may create the following adverse effects:

■

negative impact on the Fund's performance;

■

potential dilution of the value of the Fund's shares;

■

interference with the efficient management of the Fund's portfolio, such as the need to maintain undesirably large cash positions, the need to use its line of credit or the need to buy or sell securities it otherwise would not have bought or sold;

■

losses on the sale of investments resulting from the need to sell securities at less favorable prices;

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■

increased taxable gains to the Fund's remaining shareholders resulting from the need to sell securities to meet sell orders; and

■

increased brokerage and administrative costs.

To the extent that the Fund invests significantly in foreign securities traded on markets that close before the Fund's valuation time, it may be particularly susceptible to dilution, which may be exacerbated as a result of excessive trading. Because events may occur after the close of foreign markets and before the Fund's valuation time that influence the value of foreign securities, investors may seek to trade Fund shares in an effort to benefit from their understanding of the value of foreign securities as of the Fund's valuation time. This is often referred to as price arbitrage. The Fund has adopted procedures designed to adjust closing market prices of foreign securities under certain circumstances to reflect what the Fund believes to be the fair value of those securities as of its valuation time. To the extent the adjustments do not work fully, investors engaging in price arbitrage may cause dilution in the value of the Fund's shares held by other shareholders.

The Columbia Acorn Funds may invest in thinly traded equity securities of small-capitalization companies. Because these securities are often traded infrequently, investors may seek to trade Fund shares in an effort to benefit from their understanding of the value of these securities as of the Fund's valuation time. This is also a type of price arbitrage. Any such frequent trading strategies may interfere with efficient management of the Fund's portfolio to a greater degree than would be the case for mutual funds that invest only, or significantly, in highly liquid securities, in part because the Fund may have difficulty selling these particular investments at advantageous times or prices to satisfy large and/or frequent sell orders. Any successful price arbitrage may also cause dilution in the value of Fund shares held by non-redeeming shareholders.

Opening an Account and Placing Orders

We encourage you to consult with a financial advisor who can help you with your investment decisions and who can help you open an account. Once you have an account, you can buy, sell or exchange shares by contacting your financial advisor who will send your order to the Transfer Agent or your financial intermediary. As described below, once you have an account you can also communicate your orders directly to the Transfer Agent by mail, by telephone or online.

The Funds are generally available directly and through broker-dealers, banks and other financial intermediaries or institutions, and through certain qualified and non-qualified plans, wrap fee products or other investment products sponsored by financial intermediaries. You may buy, sell, or exchange shares through your financial intermediary. If you maintain your account directly with your financial intermediary, you must contact that agent to process your transaction.

Not all financial intermediaries offer the Funds (or all classes of Fund shares) and certain financial intermediaries that offer the Funds may not offer all Funds on all investment platforms or programs. Please consult with your financial intermediary to determine the availability of the Funds. If you set up an account at a financial intermediary that does not have, and is unable to obtain, a selling agreement with the Distributor, you will not be able to transfer Fund holdings to that account. In that event, you must either maintain your Fund holdings with your current financial intermediary or find another financial intermediary with a selling agreement.

Financial intermediaries that offer the Funds may charge you additional fees for the services they provide and they may have different policies that are not described in this prospectus. An investor transacting in a class of Fund shares without any front-end sales charge, CDSC, or other asset-based fee for sales or distribution, such as a Rule 12b-1 fee, may be required to pay a commission to the financial intermediary for effecting such transactions. The Funds are offered in a number of different share classes that have different fees and expenses and other features. Some differences in the policies of different financial intermediaries may include different minimum investment amounts, exchange privileges, Fund/class choices and cutoff times for investments. Additionally, recordkeeping, transaction processing and payments of distributions relating to your account may be performed by the financial intermediaries through which your shares of the Fund are held. Since the Fund (and its service providers) may not have a record of your account transactions, you should always contact the financial intermediary through which you purchased or at which you maintain your shares of the Fund to make changes to your account, to give instructions

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Buying, Selling and Exchanging Shares (continued)

concerning your account, or to obtain information about your account. The Fund and its service providers, including the Distributor and the Transfer Agent, are not responsible for the failure of any financial intermediary to carry out its obligations to its customers.

The Fund may engage financial intermediaries to receive purchase, exchange and sell orders on its behalf. Accounts established directly with the Fund will be serviced by the Transfer Agent. The Funds, the Transfer Agent and the Distributor do not provide investment advice.

Direct-At-Fund Accounts (Accounts Held Directly with the Fund)

Fund shares can be held in a variety of ways. You can hold Fund shares through an account established and held through the financial intermediary through which you purchased Fund shares, or you or your financial intermediary can establish an account directly with the Fund, in which case you will receive Fund account transaction confirmations and statements from the Transfer Agent, and not your financial intermediary (Direct-at-Fund Accounts). Direct-at-Fund Accounts include accounts held at the Transfer Agent that do not or no longer have a financial intermediary assigned to them.

To open a Direct-at-Fund Account, complete a Fund account application with your financial advisor or investment professional, and mail the account application to the Transfer Agent. Account applications may be obtained at columbiathreadneedleus.com or may be requested by calling 800.345.6611. Make your check payable to the Fund. You will be assessed a $15 fee for any checks rejected by your financial institution due to insufficient funds or other reasons. The Funds do not accept cash, credit card convenience checks, money orders, traveler's checks, starter checks, third or fourth party checks, or other cash equivalents.

Mail your check and completed application to the Transfer Agent at its regular or express mail address that can be found at the beginning of the section Choosing a Share Class. You may also use these addresses to request an exchange or redemption of Fund shares. When a written order to buy, sell or exchange shares is sent to the Transfer Agent, the share price used to fill the order is the next price calculated by the Fund after the Transfer Agent receives your transaction request in “good form” at its transaction processing center (i.e., the Fund’s express mail address), not the P.O. Box provided for regular mail delivery.

You will be sent a statement confirming your purchase and any subsequent transactions in your account. You will also be sent quarterly and annual statements detailing your transactions in the Fund and the other Funds you own under the same account. Duplicate quarterly account statements for the current year and duplicate annual statements for the most recent prior calendar year will be sent to you free of charge. Copies of year-end statements for prior years are available for a fee. Please contact the Transfer Agent for more information.

Written Transactions – Direct-at-Fund Accounts

If you have a Direct-at-Fund Account, you can communicate written buy, sell or exchange orders to the Transfer Agent at its address that can be found at the beginning of the section Choosing a Share Class. When a written order to buy, sell or exchange shares is sent to the Transfer Agent, the share price used to fill the order is the next price calculated by the Fund after the Transfer Agent receives your transaction request in “good form” at its transaction processing center (i.e., the Fund’s express mail address), not the P.O. Box provided for regular mail delivery.

Include in your transaction request letter: your name; the name of the Fund(s); your account number; the class of shares to be purchased, exchanged or sold; your SSN or other TIN; the dollar amount or number of shares you want to purchase, exchange or sell; specific instructions regarding delivery of any redemption proceeds or exchange destination (i.e., the Fund/class to be exchanged into); signature(s) of all registered account owner(s); and any special documents the Transfer Agent may require in order to process your order.

Corporate, trust or partnership accounts may need to send additional documents. Payment will be mailed to the address of record and made payable to the names listed on the account, unless your request specifies differently and is signed by all owners.

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Telephone Transactions – Direct-at-Fund Accounts

For Class A, Class C, Class Inst, Class Inst3 and (only for redemptions) Class S shares, if you have a Direct-at-Fund Account, you may place orders to buy, sell or exchange shares by telephone through the Transfer Agent. To place orders by telephone, call 800.422.3737. Have your account number and SSN or TIN available when calling.

You can sell Fund shares via telephone and receive redemption proceeds: by electronic funds transfer via ACH, by wire, or by check to the address of record, subject to a maximum of $100,000 of shares per day, per Fund account. You can buy Fund shares via telephone by electronic funds transfer via ACH from your bank account up to a maximum of $100,000 of shares per day, per Fund account, or by wire from your bank account without a maximum. See below for more information regarding wire and electronic fund transfer transactions. Certain restrictions apply, so please call the Transfer Agent at 800.422.3737 for this and other information in advance of any need to transact via telephone.

Telephone orders may not be as secure as written orders. The Fund will take reasonable steps to confirm that telephone instructions are genuine. For example, we require proof of your identification before we will act on instructions received by telephone and may record telephone conversations. However, the Fund and its agents will not be responsible for any losses, costs or expenses resulting from an unauthorized telephone instruction when reasonable steps have been taken to confirm that telephone instructions are genuine. Telephone orders may be difficult to complete during periods of significant economic or market change or business interruption.

Online Transactions – Direct-at-Fund Accounts

For Class A, Class C, Class Inst, Class Inst3 and (only for redemptions) Class S shares, if you have a Direct-at-Fund Account, you may be able to place orders to buy, sell, or exchange shares online. Contact the Transfer Agent at 800.345.6611 for more information on certain account trading restrictions and the special sign-up procedures required for online transactions. You can also go to columbiathreadneedleus.com/investor/ to sign up for online transactions. The Transfer Agent has procedures in place to authenticate electronic orders you send through the internet. You will be required to accept the terms of an online agreement and to establish an online account and utilize a password in order to access online account services. You can sell a maximum of $100,000 of shares per day, per Fund account through your online account if you qualify for internet orders. Wire transactions are not permitted online.

Wire Transactions – Direct-at-Fund Accounts

If you hold a Direct-at-Fund Account, you may purchase or redeem Class A, Class C, Class Inst, Class Inst3 and (only for redemptions) Class S shares of a Fund by wiring money from (or to) your bank account to (or from) your Fund account. You must set up this feature prior to your request unless you are submitting your request in writing, which may require a Medallion Signature Guarantee. Please contact the Transfer Agent by calling 800.422.3737 to obtain the necessary forms and requirements. The Transfer Agent charges a fee for shares sold by wire. The Transfer Agent may waive the fee for certain accounts. In the case of a redemption, the receiving bank may charge an additional fee. The minimum amount that can be redeemed by wire is $500. When selling Fund shares via a telephone request, the maximum amount that can be redeemed via wire transfer is $100,000 per day, per Fund account. Wire transactions are not permitted online.

Electronic Funds Transfer via ACH – Direct-at-Fund Accounts

If you hold a Direct-at-Fund Account, you may purchase or redeem Class A, Class C, Class Inst, Class Inst3 and (only for redemptions) Class S shares of a Fund by electronically transferring money via Automated Clearing House (ACH) from (or to) your bank account to (or from) your Fund account subject to a maximum of $100,000 of shares per day, per Fund account. You must set up this feature prior to your request, unless you are submitting your request in writing, which may require a Medallion Signature Guarantee. Please contact the Transfer Agent by calling 800.422.3737 to obtain the necessary forms and requirements. Your bank may take up to three business days to post an electronic funds transfer to (or from) your Fund account.

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Buying, Selling and Exchanging Shares (continued)

Buying Shares

Eligible Investors

Class A Shares

Class A shares are available to the general public for investment.

Class Adv Shares

Class Adv shares are available only to (i) omnibus retirement plans, including self-directed brokerage accounts within omnibus retirement plans that clear through institutional no transaction fee (NTF) platforms, (ii) trust companies or similar institutions, (iii) broker-dealers, banks, trust companies and similar institutions that clear Fund share transactions for their client or customer investment advisory or similar accounts through designated financial intermediaries and their mutual fund trading platforms that have been granted specific written authorization from the Transfer Agent with respect to Class Adv eligibility apart from selling, servicing or similar agreements, (iv) 501(c)(3) charitable organizations, (v) 529 plans, (vi) health savings accounts, (vii) investors participating in a fee-based advisory program sponsored by a financial intermediary or other entity that is not compensated by the Fund for those services, other than payments for shareholder servicing or sub-accounting performed in place of the Transfer Agent, and (viii) commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Adv shares within such platform.

Class C Shares

Class C shares are available to the general public for investment, except that Direct-at-Fund Accounts that do not have a financial intermediary assigned to them are not permitted to purchase Class C shares; Class C share purchase orders received from Direct-at-Fund Accounts that do not have a financial intermediary assigned to the account will automatically be invested in Class A shares of the same Fund.

Class Inst Shares

Class Inst shares are available only to the categories of eligible investors described below under Class Inst Shares Minimum Initial Investments. Financial intermediaries that clear Fund share transactions through designated financial intermediaries and their mutual fund trading platforms that were given specific written notice from the Transfer Agent of the termination, effective March 29, 2013, of their eligibility for new purchases of Class Inst shares and omnibus retirement plans are not permitted to establish new Class Inst accounts, subject to certain exceptions described below.

Omnibus retirement plans that opened and, subject to certain exceptions, funded a Class Inst account with the Fund as of the close of business on March 28, 2013 and have continuously held Class Inst shares in such account after such date (each, a grandfathered plan), may generally continue to make additional purchases of Class Inst shares, open new Class Inst accounts and add new participants. In addition, an omnibus retirement plan affiliated with a grandfathered plan may, in the sole discretion of the Distributor, open new Class Inst accounts in a Fund if the affiliated plan opened a Class Inst account on or before March 28, 2013. If an omnibus retirement plan invested in Class Inst shares changes recordkeepers after March 28, 2013, any new accounts established for that plan may not be established in Class Inst shares, but such a plan may establish new accounts in a different share class for which the plan is eligible.

Accounts of financial intermediaries (other than omnibus retirement plans, which are discussed above) that clear Fund share transactions for their client or customer accounts through designated financial intermediaries and their mutual fund trading platforms that received specific written notice from the Transfer Agent of the termination, effective March 29, 2013, of their eligibility for new purchases of Class Inst shares will not be permitted to establish new Class Inst accounts or make additional purchases of Class Inst shares (other than through reinvestment of distributions). Any such account may, at its holder’s option, exchange Class Inst shares of a Fund, without the payment of a sales charge, for Class A shares of the same Fund.

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Class Inst2 Shares

Class Inst2 shares are available only to (i) certain registered investment advisers and family offices that clear Fund share transactions for their client or customer accounts through designated financial intermediaries and their mutual fund trading platforms that have been granted specific written authorization from the Transfer Agent with respect to Class Inst2 eligibility apart from selling, servicing or similar agreements; (ii) omnibus retirement plans; (iii) health savings accounts, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst2 shares within such platform and that Fund shares are held in an omnibus account; and (iv) institutional investors that are clients of the Columbia Threadneedle Global Institutional Distribution Team that invest in Class Inst2 shares for their own account through platforms approved by the Distributor or an affiliate thereof to offer and/or service Class Inst2 shares within such platform. Prior to November 8, 2012, Class Inst2 shares were closed to new investors and new accounts, subject to certain exceptions. Existing shareholders who do not satisfy the new eligibility requirements for investment in Class Inst2 may not establish new Class Inst2 accounts but may continue to make additional purchases of Class Inst2 shares in accounts opened and funded prior to November 8, 2012; provided, however, that investment advisory programs and similar programs that opened a Class Inst2 account as of May 1, 2010, and continuously hold Class Inst2 shares in such account after such date, may generally not only continue to make additional purchases of Class Inst2 shares but also open new Class Inst2 accounts for such pre-existing programs and add new shareholders in the program.

Class Inst3 Shares

Class Inst3 shares are available to: (i) group retirement plans that maintain plan-level or omnibus accounts with the Fund (through the Transfer Agent); (ii) institutional investors that are clients of the Columbia Threadneedle Global Institutional Distribution Team that invest in Class Inst3 shares for their own account through platforms approved by the Distributor or an affiliate thereof to offer and/or service Class Inst3 shares within such platform; (iii) collective trust funds; (iv) affiliated or unaffiliated mutual funds (e.g., funds operating as funds-of-funds); (v) fee-based platforms of financial intermediaries (or the clearing intermediary that they trade through) that have an agreement with the Distributor or an affiliate thereof that specifically authorizes the financial intermediary to offer and/or service Class Inst3 shares within such platform, provided also that Fund shares are held in an omnibus account; (vi) commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst3 shares within such platform and that Fund shares are held in an omnibus account; (vii) health savings accounts, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst3 shares within such platform and that Fund shares are held in an omnibus account; and (viii) bank trust departments, subject to an agreement with the Distributor that specifically authorizes offering Class Inst3 shares and provided that Fund shares are held in an omnibus account. In each case above where noted that Fund shares are required to be held in an omnibus account, the Distributor may, in its discretion, determine to waive this requirement.

Class S Shares

Effective on or about October 1, 2024, Class S shares are available (i) to be held, but not purchased, by accounts of financial intermediaries that, with specific written permission from the Transfer Agent, have instructed the Fund to exchange Class Inst shares for Class S shares; and (ii) to be held and purchased by omnibus group retirement plans that (A) opened and, subject to exceptions, funded a Class Inst share account as of the close of business on March 28, 2013 and have continuously held Class Inst shares in such account through at least September 30, 2024 and (B) received specific written permission from the Transfer Agent to exchange Class Inst shares for Class S shares. Only omnibus retirement plans referenced in (ii) above may purchase Class S shares, open new Class S accounts, or add new plan participants to a Class S shares omnibus retirement plan account. Otherwise, Class S shares are not available for purchase (other than through the reinvestment of distributions on Class S shares).

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Buying, Selling and Exchanging Shares (continued)

Additional Eligible Investors

In addition, the Distributor, in its sole discretion, may accept investments in any share class from investors other than those listed in this prospectus, and may also waive certain eligibility requirements for operational and other reasons, including but not limited to any requirement to maintain Fund shares in networked or omnibus accounts.

Minimum Initial Investments

The table below shows the Fund’s minimum initial investment requirements, which may vary by class and type of account.

The Fund reserves the right to redeem your shares if your account falls below the Fund’s minimum initial investment requirement.

Minimum Initial Investments
	Minimum Initial Investment(a)	Minimum Initial Investment for Accounts with Systematic Investment Plans
For all classes and account types except those listed below	$2,000	$100
Individual Retirement Accounts for all classes except those listed below	$1,000	$100
Group retirement plans	None	N/A
Class Adv and Class Inst	$0, $1,000 or $2,000(b)	$100(b)
Class Inst2	None	N/A
Class Inst3	$0, $1,000, $2,000 or $1 million(c)	$100(c)

(a)

If your Class A, Class Adv, Class C, Class Inst, or Class Inst3 shares account balance falls below the minimum initial investment amount for any reason, including a market decline, you may be asked to increase it to the minimum initial investment amount or establish a monthly Systematic Investment Plan. If you do not do so, your account will be subject to a $20 annual low balance fee and/or shares may be automatically redeemed and the proceeds mailed to you if the account falls below the minimum account balance. See Buying, Selling and Exchanging Shares — Transaction Rules and Policies above. There is no minimum initial investment in Class A shares for accounts held in an omnibus account on a mutual fund only platform offered through your financial intermediary.

(b)

The minimum initial investment in Class Adv shares is $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts) for commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customers, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Adv shares within such platform; for all other eligible Class Adv share investors (see Buying Shares – Eligible Investors – Class Adv Shares above), there is no minimum initial investment. The minimum initial investment amount for Class Inst shares is $0, $1,000 or $2,000 depending upon the category of eligible investor. See — Class Inst Shares Minimum Initial Investments below. The minimum initial investment amount for monthly Systematic Investment Plan accounts is the same as the amount set forth in the first two rows of the table, as applicable.

(c)

There is no minimum initial investment in Class Inst3 shares for: group retirement plans that maintain plan-level or omnibus accounts with the Fund; collective trust funds; affiliated or unaffiliated mutual funds (e.g., funds operating as funds-of-funds); and fee -based platforms of financial intermediaries (or the clearing intermediary that they trade through) that have an agreement with the Distributor or an affiliate thereof that specifically authorizes the financial intermediary to offer and/or service Class Inst3 shares within such platform and Fund shares are held in an omnibus account. The minimum initial investment in Class Inst3 shares is $2,000 ($1,000 for IRAs; $100 for monthly Systematic Investment Plan accounts) for commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst3 shares within such platform and Fund shares are held in an omnibus account. The minimum initial investment in Class Inst3 shares is $1 million, unless waived in the discretion of the Distributor, for the following investors: institutional investors that are clients of the Columbia Threadneedle Global Institutional Distribution Team that invest in Class Inst3 shares for their own account through platforms approved by the Distributor or an affiliate thereof to offer and/or service Class Inst3 shares within such platform; and bank trust departments, subject to an agreement with the Distributor that specifically authorizes offering Class Inst3 shares and provided that Fund shares are held in an omnibus account. The Distributor may, in its discretion, waive the $1 million minimum initial investment required for these Class Inst3 investors. In each case above where noted that Fund shares are required to be held in an omnibus account, the Distributor, may, in its discretion, determine to waive this requirement.

Additional Information about Minimum Initial Investments

The minimum initial investment requirements may be waived for accounts that are managed by an investment professional, or for accounts held in approved discretionary or non-discretionary wrap programs. The Distributor, in its sole discretion, may also waive minimum initial investment requirements for other account types.

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

Minimum investment and related requirements may be modified at any time, with or without prior notice. If your account is closed and then re-opened with a Systematic Investment Plan, your account must meet the then-current applicable minimum initial investment.

Class Inst Shares Minimum Initial Investments

There is no minimum initial investment in Class Inst shares for the following categories of eligible investors:

■

Any health savings account sponsored by a third party platform.

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Any investor participating in an account sponsored by a financial intermediary or other entity (that provides services to the account) that is paid a fee-based advisory fee by the investor and that is not compensated by the Fund for those services, other than payments for shareholder servicing or sub-accounting performed in place of the Transfer Agent.

■

Any commissionable brokerage account, if a financial intermediary has received a written approval from the Distributor to waive the minimum initial investment in Class Inst shares.

■

Any Trustee (or family member) of Columbia Acorn Trust.

■

Any employee (or family member) of the Investment Manager.

The minimum initial investment in Class Inst shares for the following categories of eligible investors is $1,000:

■

Individual retirement accounts (IRAs) on commissionable brokerage platforms where the financial intermediary, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares, provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst shares within such platform.

■

Any current employee of Columbia Management Investment Advisers LLC, the Distributor or the Transfer Agent and immediate family members of any of the foregoing who share the same address are eligible to invest in Class Inst shares through an individual retirement account (IRA). If you maintain your account with a financial intermediary, you must contact that financial intermediary each time you seek to purchase shares to notify them that you qualify for Class Inst shares. If Class Inst shares are not available at your financial intermediary, you may consider opening a Direct-at-Fund Account. It is your obligation to advise your financial intermediary or (in the case of Direct-at-Fund Accounts) the Transfer Agent that you qualify for Class Inst shares; be prepared to provide proof thereof.

The minimum initial investment in Class Inst shares for the following categories of eligible investors is $2,000:

■

Investors (except investors in individual retirement accounts (IRAs)) who purchase Fund shares through commissionable brokerage platforms where the financial intermediary holds the shares in an omnibus account and, acting as broker on behalf of its customer, charges the customer a commission for effecting transactions in Fund shares provided that the financial intermediary has an agreement with the Distributor that specifically authorizes offering Class Inst shares within such platform.

■

Any current employee of Columbia Management Investment Advisers LLC, the Distributor or the Transfer Agent and immediate family members of any of the foregoing who share the same address are eligible to invest in Class Inst shares (other than individual retirement accounts (IRAs), for which the minimum initial investment is $1,000). If you maintain your account with a financial intermediary, you must contact that financial intermediary each time you seek to purchase shares to notify them that you qualify for Class Inst shares. If Class Inst shares are not available at your financial intermediary, you may consider opening a Direct-at-Fund Account. It is your obligation to advise your financial intermediary or (in the case of Direct-at-Fund Accounts) the Transfer Agent that you qualify for Class Inst shares; be prepared to provide proof thereof.

■

Certain financial institutions and intermediaries, such as insurance companies, trust companies, banks, endowments, investment companies or foundations, buying shares for their own account, including Ameriprise Financial and its affiliates and/or subsidiaries.

■

Bank trust departments that assess their clients an asset-based fee.

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Any trustee or director (or family member of a trustee or director) of a fund distributed by the Distributor (other than the Columbia Acorn Funds).

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

■

Certain other investors as set forth in more detail in the SAI.

Systematic Investment Plan

The Systematic Investment Plan allows you to schedule regular purchases via automatic transfers from your bank account to the Fund on a monthly, quarterly or semiannual basis. Contact the Transfer Agent or your financial intermediary to set up the plan. Systematic Investment Plans may not be available for all share classes. With the exception of Columbia Government Money Market Fund, Systematic Investment Plan purchases will be confirmed on your quarterly account statement. Columbia Government Money Market Fund Systematic Investment Plan purchases will be confirmed at the time of the purchase. If establishing a Systematic Investment Plan to reduce the initial fund minimum, the plan needs to be established monthly to avoid the small account fee.

Dividend Diversification

Generally, you may automatically invest Fund distributions into the same class of shares (and in some cases certain other classes of shares) of another Fund without paying any applicable front-end sales charge. Call the Transfer Agent at 800.345.6611 for details. The ability to invest distributions from one Fund to another Fund may not be available to accounts held at all financial intermediaries.

Other Purchase Rules You Should Know

■

Once the Transfer Agent or your financial intermediary receives your purchase order in “good form,” your purchase will be made at the Fund’s next calculated public offering price per share, which is the NAV per share plus any sales charge that applies (i.e., the trade date).

■

Once the Fund receives your purchase request in “good form,” you cannot cancel it after the market closes.

■

You generally buy Class A shares at the public offering price per share because purchases of these share classes are generally subject to a front-end sales charge.

■

Subject to Class eligibility, you buy Class Adv, Class C, Class Inst, Class Inst2, Class Inst3 and Class S shares at NAV per share because no front-end sales charge applies to purchases of these share classes.

■

The Distributor and the Transfer Agent reserve the right to cancel your order request if the Fund does not receive payment within two business days of receiving your purchase order request. The Fund will return any payment received for orders that have been cancelled, but no interest will be paid on that money.

■

Financial intermediaries are responsible for sending your purchase orders to the Transfer Agent and ensuring that the Fund receives your money on time.

■

Shares purchased are recorded on the books of the Fund. The Fund does not issue certificates.

■

You generally may make a purchase only into a Fund that is accepting investments.

Please also read Appendix A and contact your financial intermediary for more information regarding any reductions and/or waivers described therein.

Selling Shares

When you sell shares, the amount you receive may be more or less than the amount you invested. Your sale price will be the next NAV calculated after your request is received in “good form,” (i.e., the trade date) minus any applicable CDSC.

Systematic Withdrawal Plan

The Systematic Withdrawal Plan allows you to schedule regular redemptions from your account any business day on a monthly, quarterly or semiannual basis. Currently, Systematic Withdrawal Plans are generally available for Class A, Class Adv, Class C, Class Inst, Class Inst2, Class Inst3 and Class S share accounts. Contact the Transfer Agent or your financial intermediary to set up the plan. To set up the plan, your account balance must meet the class minimum initial investment amount. A Systematic Withdrawal Plan cannot be set up on an account that already has a Systematic Investment Plan established. Note that a Medallion Signature Guarantee may be required if this service is established after your Fund account is opened.

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

You can choose to receive your withdrawals via check or direct deposit into your bank account. The Fund will deduct any applicable CDSC from the withdrawals before sending redemption proceeds to you. You can cancel the plan by giving the Fund 30 days’ notice in writing or by calling the Transfer Agent at 800.422.3737. It’s important to remember that if you withdraw more than your investment in the Fund is earning, you'll eventually withdraw your entire investment.

Satisfying Fund Redemption Requests

When you sell your Fund shares, the Fund is effectively buying them back from you. This is called a redemption. Except as noted below with respect to newly purchased shares, the Fund typically expects to send you payment for your shares within two business days after your trade date. The Fund can suspend redemptions and/or delay payment of redemption proceeds for up to seven days. The Fund can also suspend redemptions and/or delay payment of redemption proceeds in excess of seven days under certain circumstances, including when the NYSE is closed or trading thereon is restricted or during emergency or other circumstances, including as determined by the SEC.

The Fund typically seeks to satisfy redemption requests from cash or cash equivalents held by the Fund, from the proceeds of orders to purchase Fund shares or from the proceeds of sales of Fund holdings effected in the normal course of managing the Fund. However, the Fund may have to sell Fund holdings, including in down markets, to meet heavier than usual redemption requests. For example, under stressed or abnormal market conditions or circumstances, including circumstances adversely affecting the liquidity of the Fund’s investments, the Fund may be more likely to be forced to sell Fund holdings to meet redemptions than under normal market circumstances. In these situations, the Fund’s portfolio manager may have to sell Fund holdings that would not otherwise be sold because, among other reasons, the current price to be received is less than the value of the holdings perceived by the Fund’s portfolio manager. The Fund may also, under certain circumstances (likely under stressed or abnormal market conditions or circumstances), borrow money under a credit facility to which the Fund and certain other Columbia Funds are parties. The Fund and the other Columbia Funds are limited as to the amount that each may individually and collectively borrow under the credit facility. As a result, borrowings available to the Fund under the credit facility might be insufficient, alone or in combination with the other strategies described herein, to satisfy Fund redemption requests. Please see About Fund Investments – Borrowings in the SAI for more information about the credit facility. The Fund is also limited in the total amount it may borrow. Please see About Fund Investments – Fundamental and Non-Fundamental Investment Policies in the SAI for more information about borrowing.

In addition, the Fund reserves the right to honor redemption orders in whole or in part with in-kind distributions of Fund portfolio securities instead of cash. Such in-kind distributions typically represent a pro-rata portion of Fund portfolio assets subject to adjustments (e.g., for non-transferable securities, round lots, and derivatives, if any). In the event the Fund distributes portfolio securities in kind, you may incur brokerage and other transaction costs associated with converting the portfolio securities you receive into cash. Also, the portfolio securities you receive may increase or decrease in value after they are distributed but before you convert them into cash. For U.S. federal income tax purposes, redemptions paid in securities are generally treated the same as redemptions paid in cash. The Trust has elected to be governed by Rule 18f-1 under the 1940 Act, which means that for each redeeming shareholder, the first $250,000, or 1% of the Fund's net asset value, whichever is less, will be paid in cash or cash equivalents and the remainder may be paid in kind.

While the Fund is not required (and may refuse in its discretion) to pay a redemption with an in-kind distribution of Fund portfolio securities and reserves the right to pay the redemption proceeds in cash, if you wish to request an in-kind redemption, please call the Transfer Agent at 800.345.6611. As a result of the operational steps needed to coordinate with the redeeming shareholder’s custodian, in-kind redemptions typically take several weeks to complete after a redemption request is received. The Fund and the redeeming shareholder will typically agree upon a redemption date. Since the Fund’s NAV may fluctuate during this time, the Fund’s NAV may be lower on the agreed-upon redemption date than on an earlier date on which the investment could have been redeemed for cash.

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

Redemption of Newly Purchased Shares

You may not redeem shares for which the Fund has not yet received payment. Shares purchased by check or electronically by ACH when the purchase payment is not guaranteed will be considered in “good form” for redemption only after they have been held in your account for 6 calendar days after the trade date of the purchase (Collected Shares). If you request a redemption for an amount that, based on the NAV next calculated after your redemption request is received, includes any shares that are not yet Collected Shares, the Fund will only process the redemption up to the amount of the value of Collected Shares available in your account. You must submit a new redemption request if you wish to redeem those shares that were not yet Collected Shares at the time the original redemption request was received by the Fund.

Other Redemption Rules You Should Know

■

Once the Transfer Agent or your financial intermediary receives your redemption order in “good form,” your shares will be sold at the Fund’s next calculated NAV per share (i.e., the trade date). Any applicable CDSC will be deducted from the amount you're selling and the balance will be remitted to you.

■

Once the Fund receives your redemption request in “good form,” you cannot cancel it after the market closes.

■

The Distributor, in its sole discretion, reserves the right to liquidate Fund shares (of any class of the Fund) held in an omnibus account of a financial intermediary that clears Fund share transactions through a clearing intermediary or platform that charges certain maintenance fees to the Fund if the value of the omnibus account, at the Fund share class (i.e., CUSIP) level, falls below $100,000 (a CUSIP Liquidation Event). The Distributor will provide at least 90 days’ notice of a CUSIP Liquidation Event to financial intermediaries with impacted omnibus accounts. Shareholders invested in the Fund through such omnibus accounts can request through their financial intermediary a tax-free exchange to Class A shares or shareholders can consider holding their Fund shares in a Direct-at-Fund Account, provided requirements to transfer the account are fulfilled. You should discuss your options with your financial intermediary.

■

If you sell your shares that are held in a Direct-at-Fund Account, we will normally send the redemption proceeds by mail or electronically transfer them to your bank account the next business day after the trade date. Note that your bank may take up to three business days to post an electronic funds transfer from your account.

■

If you sell your shares through a financial intermediary, the Funds will normally send the redemption proceeds to your financial intermediary within two business days after the trade date.

■

No interest will be paid on uncashed redemption checks.

■

Other restrictions may apply to retirement accounts. For information about these restrictions, contact your retirement plan administrator.

■

For broker-dealer and wrap fee accounts: The Fund reserves the right to redeem your shares if your account falls below the Fund's minimum initial investment requirement. The Fund will notify your broker-dealer prior to redeeming shares, and will provide details on how to avoid such redemption.

■

Also keep in mind the Funds' Small Account Policy, which is described above in Buying, Selling and Exchanging Shares — Transaction Rules and Policies.

Exchanging Shares

You can generally sell shares of your Fund to buy shares of another Fund (subject to eligibility requirements), in what is called an exchange. You should read the prospectus of, and make sure you understand the investment objective, principal investment strategies, risks, fees and expenses of, the Fund into which you are exchanging. Although the Funds allow certain exchanges from one share class to another share class with higher expenses, you should consider the expenses of each class before making such an exchange. Please see Same-Fund Exchange Privilege below for more information.

You will be subject to a sales charge if, in a Direct-at-Fund Account, you exchange shares that have not previously paid a sales charge, including from Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia U.S.

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

Treasury Index Fund or any other Columbia Fund that does not charge a front-end sales charge, into a Columbia Fund that does assess a sales charge. If you hold your Fund shares through certain financial intermediaries, you may have limited exchangeability among the Funds. Please contact your financial intermediary for more information.

Systematic Exchanges

You may buy Class A, Class C, Class Inst and Class Inst3 shares of a Fund by exchanging each month from another Fund for shares of the same class of the Fund at no additional cost, subject to the following exchange amount minimums: $50 each month for individual retirement accounts (i.e., tax qualified accounts); and $100 each month for non-retirement accounts. Contact the Transfer Agent or your financial intermediary to set up the plan.

Exchanges will continue as long as your balance in the Fund you are exchanging shares from is sufficient to complete the systematic monthly exchange, subject to the Funds' Small Account Policy described above in Buying, Selling and Exchanging Shares — Transaction Rules and Policies. You may terminate the program or change the amount you would like to exchange (subject to the $50 and $100 minimum requirements noted immediately above) by calling the Transfer Agent at 800.345.6611.

Other Exchange Rules You Should Know

■

Exchanges are made at the NAV next calculated (plus any applicable sales charge) after your exchange order is received in “good form” (i.e., the trade date).

■

Once the Fund receives your exchange request in “good form,” you cannot cancel it after the market closes.

■

The rules for buying shares of a Fund generally apply to exchanges into that Fund, including, if your exchange creates a new Fund account, it must satisfy the minimum investment amount, unless a waiver applies.

■

Shares of the purchased Fund may not be used on the same day for another exchange or sale.

■

A sales charge may apply when you exchange shares of a Fund that were not assessed a sales charge at the time you purchased such shares. If you invest through a Direct-at-Fund Account in Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration Municipal Bond Fund, Columbia Ultra Short Term Bond Fund, Columbia U.S. Treasury Index Fund or any other Columbia Fund that does not impose a front-end sales charge and then you exchange into a Fund that does assess a sales charge, your transaction is subject to a front-end sales charge if you exchange into Class A shares and to a CDSC if you exchange into Class C shares of another Columbia Fund.

■

If you purchased Class A shares of a Columbia Fund that imposes a front-end sales charge (and you paid any applicable sales charge) and you then exchange those shares into Columbia Government Money Market Fund, Columbia Large Cap Enhanced Core Fund, Columbia Large Cap Index Fund, Columbia Mid Cap Index Fund, Columbia Small Cap Index Fund, Columbia Ultra Short Duration Municipal Bond Fund, Columbia Ultra Short Term Bond Fund, Columbia U.S. Treasury Index Fund or any other Columbia Fund that does not impose a front-end sales charge, you may exchange that amount to Class A of another Columbia Fund in the future, including dividends earned on that amount, without paying a sales charge.

■

If your shares are subject to a CDSC, you will not be charged a CDSC upon the exchange of those shares. Any CDSC will be deducted when you sell the shares you received from the exchange. The CDSC imposed at that time will be based on the period that begins when you bought shares of the original Fund and ends when you sell the shares of the Fund you received from the exchange. Any applicable CDSC charged will be the CDSC of the original Fund.

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You may make exchanges only into a Fund that is legally offered and sold in your state of residence. Contact the Transfer Agent or your financial intermediary for more information.

■

You generally may make an exchange only into a Fund that is accepting investments.

■

The Fund may change or cancel your right to make an exchange by giving the amount of notice required by regulatory authorities (generally 60 days for a material change or cancellation).

Prospectus 2024

Columbia Acorn® Fund

Buying, Selling and Exchanging Shares (continued)

■

Unless your account is part of a tax-advantaged arrangement, an exchange for shares of another Fund is a taxable event, and you may recognize a gain or loss for tax purposes.

■

Changing your investment to a different Fund will be treated as a sale and purchase, and you will be subject to applicable taxes on the sale and sales charges on the purchase of the new Fund.

■

Class Inst shares of a Fund may be exchanged for Class A or Class Inst shares of another Fund. In certain circumstances, the front-end sales charge applicable to Class A shares may be waived on exchanges of Class Inst shares for Class A shares. See Buying, Selling and Exchanging Shares — Buying Shares — Eligible Investors — Class Inst Shares for details.

Same-Fund Exchange Privilege

Shareholders may be eligible to invest in other classes of shares of the same Fund, and may exchange their current shares for another share class if deemed eligible and offered by the Fund. Such same-Fund exchanges could include an exchange of one class for another with higher expenses. Before making such an exchange, you should consider the expenses of each class. Shareholders should contact their financial intermediaries to learn more about the details of the same-Fund exchange privilege. Exchanges out of Class A and Class C shares will be subject to any applicable CDSC. Financial intermediaries that have a customized arrangement with regard to CDSCs are detailed in Appendix A. To exchange to another class of shares of the same Fund you must meet the new class’s eligibility criteria as described in this Prospectus.

Exchanges out of Class C shares to another share class of the same Fund are not permissible on Direct-at-Fund Accounts, except that the Transfer Agent seeks to convert Class C shares as soon as administratively feasible to Class A shares of the same Fund for Direct-at-Fund Accounts that do not or no longer have a financial intermediary assigned to them. Direct-at-Fund Accounts that do not have a financial intermediary assigned to them are not permitted to purchase Class C shares. Class C share purchase orders received by Direct-at-Fund Accounts that do not have a financial intermediary assigned to the account will automatically be invested in Class A shares of the same Fund. Exchanges out of Class C shares to another share class of the same Fund within commissionable brokerage accounts are permitted only (1) when the shareholder moves from a commissionable brokerage account to a fee-based advisory program or (2) when the exchange is part of a share class conversion (or a similar multiple shareholder transaction event) instituted by a financial intermediary and such conversion or similar type event is preapproved by the Distributor.

Ordinarily, shareholders will not recognize a gain or loss for U.S. federal income tax purposes upon a same-Fund exchange. You should consult your tax advisor about your particular exchanges.

Prospectus 2024

Columbia Acorn® Fund

Distributions and Taxes

Distributions to Shareholders

A mutual fund can make money two ways:

■

It can earn income on its investments. Examples of fund income are interest paid on money market instruments and bonds, and dividends paid on common stocks.

■

A mutual fund can also have capital gains if the value of its investments increases. While a fund continues to hold an investment, any gain is generally unrealized. If the fund sells an investment, it generally will realize a capital gain if it sells that investment for a higher price than its adjusted cost basis, and will generally realize a capital loss if it sells that investment for a lower price than its adjusted cost basis. Capital gains and losses are either short-term or long-term, depending on whether the fund holds the securities for one year or less (short-term) or more than one year (long-term).

Mutual funds make payments of fund earnings to shareholders, distributing them among all shareholders of the fund. As a shareholder, you are entitled to your portion of a fund's distributed income, including capital gains. Reinvesting your distributions buys you more shares of a fund — which lets you take advantage of the potential for compound growth. Putting the money you earn back into your investment means it, in turn, may earn even more money (or be exposed to additional losses, if the Fund earns a negative return). Over time, the power of compounding has the potential to significantly increase the value of your investment. There is no assurance, however, that you'll earn more money if you reinvest your distributions rather than receive them in cash.

The Fund intends to pay out, in the form of distributions to shareholders, a sufficient amount of its income and gains so that the Fund will qualify for treatment as a regulated investment company and generally will not have to pay any federal income or excise tax. The Fund generally intends to distribute any net realized capital gain (whether long-term or short-term gain) at least once a year. Normally, the Fund will declare and pay distributions of net investment income according to the following schedule:

Declaration and Distribution Schedule
Declarations	Semiannually
Distributions	Semiannually

The Fund may declare or pay distributions of net investment income more frequently.

Different share classes of the Fund usually pay different net investment income distribution amounts, because each class has different expenses. Each time a distribution is made, the NAV per share of the share class is reduced by the amount of the distribution.

The Fund generally pays cash distributions within five business days after the distribution was declared (or, if the Fund declares distributions daily, within five business days after the end of the month in which the distribution was declared). If you sell all of your shares after the record date, but before the payment date, for a distribution, you'll normally receive that distribution in cash within five business days after the sale was made.

The Fund will automatically reinvest distributions in additional shares of the same share class of the Fund unless you inform us you want to receive your distributions in cash (the financial intermediary through which you purchased shares may have different policies). You can do this by contacting the Funds at the addresses and telephone numbers listed at the beginning of the section entitled Choosing a Share Class. No sales charges apply to the purchase or sale of such shares.

For accounts held directly with the Fund (through the Transfer Agent), distributions of $10 or less will automatically be reinvested in additional Fund shares only. If you elect to receive distributions by check and the check is returned as undeliverable, all subsequent distributions will be reinvested in additional shares of the Fund.

Unless you are a tax-exempt investor or holding Fund shares through a tax-advantaged account (such as a 401(k) plan or IRA), you should consider avoiding buying Fund shares shortly before the Fund makes a distribution (other than distributions of net investment income that are declared daily) of net investment income or net realized capital gain, because doing so can cost you money in taxes to the extent the distribution consists of taxable income or gains. This is because you will, in effect, receive part of your purchase price back in the distribution. This is known as

Prospectus 2024

Columbia Acorn® Fund

Distributions and Taxes (continued)

“buying a dividend.” To avoid “buying a dividend,” before you invest in a Fund, check the Fund's distribution schedule, which is available at the Columbia Funds' website, columbiathreadneedleus.com, and/or by calling the Funds' telephone number listed at the beginning of the section entitled Choosing a Share Class.

Taxes

You should be aware of the following considerations applicable to the Fund:

■

The Fund intends to qualify and to be eligible for treatment each year as a regulated investment company. A regulated investment company generally is not subject to tax at the fund level on income and gains from investments that are distributed to shareholders. However, the Fund's failure to qualify for treatment as a regulated investment company would result in Fund-level taxation, and consequently, a reduction in income available for distribution to you and in the NAV of your shares. Even if the Fund qualifies for treatment as a regulated investment company, the Fund may be subject to federal excise tax on certain undistributed income or gains.

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Otherwise taxable distributions generally are taxable to you when paid, whether they are paid in cash or automatically reinvested in additional Fund shares. Dividends paid in January are deemed paid on December 31 of the prior year if the dividend was declared and payable to shareholders of record in October, November, or December of such prior year.

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Distributions of the Fund's ordinary income and net short-term capital gain, if any, generally are taxable to you as ordinary income. Distributions of the Fund's net long-term capital gain, if any, generally are taxable to you as long-term capital gain. Whether capital gains are long-term or short-term is determined by how long the Fund has owned the investments that generated them, rather than how long you have owned your shares. Certain events may require the Fund to sell significant amounts of appreciated securities and make large dividends relative to the Fund’s NAV. Such events may include large net shareholder redemptions, portfolio rebalancing or fund mergers. The Fund generally provides estimates of expected capital gain dividends (if any) prior to the distribution on columbiathreadneedleus.com.

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From time to time, a distribution from the Fund could constitute a return of capital. A return of capital is a return of an amount of your original investment and is not a distribution of income or capital gain from the Fund. Therefore, a return of capital is not taxable to you so long as the amount of the distribution does not exceed your tax basis in your Fund shares. A return of capital reduces your tax basis in your Fund shares, with any amounts exceeding such basis generally taxable as capital gain.

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If you are an individual and you meet certain holding period and other requirements for your Fund shares, a portion of your distributions may be treated as “qualified dividend income” taxable at the lower net long-term capital gain rates instead of the higher ordinary income rates. Qualified dividend income is income attributable to the Fund's dividends received from certain U.S. and foreign corporations, as long as the Fund meets certain holding period and other requirements for the stock producing such dividends.

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Certain high-income individuals (as well as estates and trusts) are subject to a 3.8% tax on net investment income. For individuals, the 3.8% tax applies to the lesser of (1) the amount (if any) by which the taxpayer's modified adjusted gross income exceeds certain threshold amounts or (2) the taxpayer's “net investment income.”

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Certain derivative instruments when held in the Fund's portfolio subject the Fund to special tax rules, the effect of which may be to, among other things, accelerate income to the Fund, defer Fund losses, cause adjustments in the holding periods of Fund portfolio securities, or convert capital gains into ordinary income, short-term capital losses into long-term capital losses or long-term capital gains into short-term capital gains. These rules could therefore affect the amount, timing and/or character of distributions to shareholders.

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Generally, a Fund realizes a capital gain or loss on an option when the option expires, or when it is exercised, sold or otherwise terminated. However, if an option is a “section 1256 contract,” which includes most traded options on a broad-based index, and the Fund holds such option at the end of its taxable year, the Fund is deemed to sell such option at fair market value at such time and recognize any gain or loss thereon, which is generally deemed to be 60% long-term and 40% short-term capital gain or loss, as described further in the SAI.

Prospectus 2024

Columbia Acorn® Fund

Distributions and Taxes (continued)

■

A sale, redemption or exchange of Fund shares is a taxable event. This includes redemptions where you are paid in securities. Your sales, redemptions and exchanges of Fund shares (including those paid in securities) usually will result in a taxable capital gain or loss to you, equal to the difference between the amount you receive for your shares (or are deemed to have received in the case of exchanges) and your adjusted tax basis in the shares, which is generally the amount you paid (or are deemed to have paid in the case of exchanges) for them. Any such capital gain or loss generally will be long-term capital gain or loss if you have held your Fund shares for more than one year at the time of sale or exchange. In certain circumstances, capital losses may be converted from short-term to long-term; in other circumstances, capital losses may be disallowed under the “wash sale” rules.

■

For sales, redemptions and exchanges of shares that were acquired in a non-qualified account after 2011, the Fund generally is required to report to shareholders and the Internal Revenue Service (IRS) cost basis information with respect to those shares. The Fund uses average cost basis as its default method of calculating cost basis. For more information regarding average cost basis reporting, other available cost basis methods, and selecting or changing to a different cost basis method, please see the SAI, columbiathreadneedleus.com, or contact the Fund at 800.345.6611. If you hold Fund shares through a financial intermediary (e.g., a brokerage firm), you should contact your financial intermediary to learn about its cost basis reporting default method and the reporting elections available to your account.

■

The Fund is required by federal law to withhold tax on any taxable or tax-exempt distributions and redemption proceeds paid to you (including amounts paid to you in securities and amounts deemed to be paid to you upon an exchange of shares) if: you have not provided a correct TIN or have not certified to the Fund that withholding does not apply, the IRS has notified us that the TIN listed on your account is incorrect according to its records, or the IRS informs the Fund that you are otherwise subject to backup withholding.

FUNDamentals

Taxes

The information provided above is only a summary of how U.S. federal income taxes may affect your investment in the Fund. It is not intended as a substitute for careful tax planning. Your investment in the Fund may have other tax implications. It does not apply to certain types of investors who may be subject to special rules, including foreign or tax-exempt investors or those holding Fund shares through a tax-advantaged account, such as a 401(k) plan or IRA. Please see the SAI for more detailed tax information. You should consult with your own tax advisor about the particular tax consequences to you of an investment in the Fund, including the effect of any foreign, state and local taxes, and the effect of possible changes in applicable tax laws.

Prospectus 2024

Columbia Acorn® Fund

Financial Highlights

The financial highlights table is intended to help you understand the Fund’s financial performance for the past five fiscal years. Certain information reflects financial results for a single Fund share. Per share net investment income (loss) amounts are calculated based on average shares outstanding during the period. The total return in the table represents the rate that an investor would have earned (or lost) on an investment in the Fund assuming all dividends and distributions had been reinvested. Total return does not reflect payment of sales charges, if any. Total return and portfolio turnover are not annualized for periods of less than one year. The ratios of expenses and net investment income are annualized for periods of less than one year. The portfolio turnover rate is calculated without regard to purchase and sales transactions of short-term instruments and certain derivatives, if any. If such transactions were included, the Fund’s portfolio turnover rate might be higher. Because Class S shares of the Fund had not commenced operations prior to the date of this prospectus, no financial highlights are provided for this class.

This information has been audited by PricewaterhouseCoopers LLP, an independent registered public accounting firm, whose report, along with the Fund’s financial statements, is included in the Fund’s annual report, which is available upon request. The independent registered public accounting firm’s report and the Fund’s financial statements are also incorporated by reference into the SAI.

Prospectus 2024

Columbia Acorn® Fund

Financial Highlights (continued)

	Net asset value, beginning of period	Net investment income (loss)	Net realized and unrealized gain (loss)	Total from investment operations	Distributions from net investment income	Distributions from net realized gains	Total distributions to shareholders
Class A
Year Ended 12/31/2023	$6.16	(0.03)	1.36	1.33	—	—	—
Year Ended 12/31/2022	$10.01	(0.04)	(3.34)	(3.38)	—	(0.47)	(0.47)
Year Ended 12/31/2021	$12.69	(0.10)	1.07	0.97	(0.14)	(3.51)	(3.65)
Year Ended 12/31/2020	$11.71	(0.06)	3.24	3.18	—	(2.20)	(2.20)
Year Ended 12/31/2019	$10.65	(0.04)	2.78	2.74	—	(1.68)	(1.68)
Advisor Class
Year Ended 12/31/2023	$9.73	(0.02)	2.15	2.13	—	—	—
Year Ended 12/31/2022	$15.37	(0.03)	(5.14)	(5.17)	—	(0.47)	(0.47)
Year Ended 12/31/2021	$17.65	(0.10)	1.54	1.44	(0.13)	(3.59)	(3.72)
Year Ended 12/31/2020	$15.52	(0.05)	4.38	4.33	—	(2.20)	(2.20)
Year Ended 12/31/2019	$13.63	(0.01)	3.58	3.57	—	(1.68)	(1.68)
Class C
Year Ended 12/31/2023	$8.61	(0.11)	1.89	1.78	—	—	—
Year Ended 12/31/2022	$13.72	(0.13)	(4.57)	(4.70)	—	(0.41)	(0.41)
Year Ended 12/31/2021	$16.14	(0.26)	1.40	1.14	(0.13)	(3.43)	(3.56)
Year Ended 12/31/2020(e)	$16.57	(0.20)	4.08	3.88	—	(4.31)	(4.31)
Year Ended 12/31/2019(e)	$18.75	(0.20)	4.74	4.54	—	(6.72)	(6.72)
Institutional Class
Year Ended 12/31/2023	$8.97	(0.02)	1.98	1.96	—	—	—
Year Ended 12/31/2022	$14.22	(0.03)	(4.75)	(4.78)	—	(0.47)	(0.47)
Year Ended 12/31/2021	$16.60	(0.09)	1.43	1.34	(0.13)	(3.59)	(3.72)
Year Ended 12/31/2020	$14.70	(0.04)	4.14	4.10	—	(2.20)	(2.20)
Year Ended 12/31/2019	$12.98	(0.01)	3.41	3.40	—	(1.68)	(1.68)
Institutional 2 Class
Year Ended 12/31/2023	$9.89	(0.02)	2.19	2.17	—	—	—
Year Ended 12/31/2022	$15.62	(0.03)	(5.23)	(5.26)	—	(0.47)	(0.47)
Year Ended 12/31/2021	$17.88	(0.10)	1.57	1.47	(0.13)	(3.60)	(3.73)
Year Ended 12/31/2020	$15.69	(0.04)	4.43	4.39	—	(2.20)	(2.20)
Year Ended 12/31/2019	$13.76	(0.00) (f)	3.61	3.61	—	(1.68)	(1.68)
Institutional 3 Class
Year Ended 12/31/2023	$10.07	(0.02)	2.23	2.21	—	—	—
Year Ended 12/31/2022	$15.88	(0.02)	(5.32)	(5.34)	—	(0.47)	(0.47)
Year Ended 12/31/2021	$18.12	(0.09)	1.60	1.51	(0.14)	(3.61)	(3.75)
Year Ended 12/31/2020	$15.87	(0.04)	4.49	4.45	—	(2.20)	(2.20)
Year Ended 12/31/2019	$13.89	0.00(f)	3.66	3.66	—	(1.68)	(1.68)

Notes to Financial Highlights
(a)	In addition to the fees and expenses that the Fund bears directly, the Fund indirectly bears a pro rata share of the fees and expenses of any other funds in which it invests, if any. Such indirect expenses are not included in the Fund's reported expense ratios.
(b)	Had the Investment Manager and/or Transfer Agent not waived fees and/or reimbursed a portion of expenses, total return would have been reduced.
(c)	The benefits derived from expense reductions had an impact of less than 0.01%.
(d)	Ratios include Trustees’ fees deferred during the current period as well as any gains or losses on the Trustees’ deferred compensation balances as a result of market fluctuations. If these had been excluded, expenses would have been higher by 0.03%.
(e)	Per share amounts have been adjusted on a retroactive basis to reflect a 4 to 1 reverse stock split completed after the close of business on September 11, 2020.
(f)	Rounds to zero.

Prospectus 2024

Columbia Acorn® Fund

Financial Highlights (continued)

	Net asset value, end of period	Total return	Total gross expense ratio to average net assets(a)	Total net expense ratio to average net assets(a)	Net investment income (loss) ratio to average net assets	Portfolio turnover	Net assets, end of period (000's)
Class A
Year Ended 12/31/2023	$7.49	21.59% (b)	1.15%	1.08% (c)	(0.46%)	64%	$570,240
Year Ended 12/31/2022	$6.16	(34.01%)	1.08% (d)	1.08% (c), (d)	(0.53%)	64%	$521,243
Year Ended 12/31/2021	$10.01	8.79%	1.08%	1.08% (c)	(0.80%)	75%	$919,643
Year Ended 12/31/2020	$12.69	29.18% (b)	1.11%	1.11% (c)	(0.56%)	86%	$983,709
Year Ended 12/31/2019	$11.71	26.21% (b)	1.11%	1.10% (c)	(0.30%)	101%	$810,966
Advisor Class
Year Ended 12/31/2023	$11.86	21.89% (b)	0.90%	0.83% (c)	(0.20%)	64%	$36,287
Year Ended 12/31/2022	$9.73	(33.79%)	0.83% (d)	0.83% (c), (d)	(0.27%)	64%	$21,946
Year Ended 12/31/2021	$15.37	9.03%	0.83%	0.83% (c)	(0.55%)	75%	$39,676
Year Ended 12/31/2020	$17.65	29.44% (b)	0.86%	0.86% (c)	(0.30%)	86%	$44,139
Year Ended 12/31/2019	$15.52	26.58% (b)	0.86%	0.85% (c)	(0.05%)	101%	$44,248
Class C
Year Ended 12/31/2023	$10.39	20.67% (b)	1.90%	1.83% (c)	(1.21%)	64%	$8,991
Year Ended 12/31/2022	$8.61	(34.48%)	1.83% (d)	1.83% (c), (d)	(1.29%)	64%	$9,153
Year Ended 12/31/2021	$13.72	7.95%	1.83%	1.83% (c)	(1.54%)	75%	$24,546
Year Ended 12/31/2020(e)	$16.14	28.27% (b)	1.86%	1.86% (c)	(1.30%)	86%	$55,861
Year Ended 12/31/2019(e)	$16.57	25.12% (b)	1.86%	1.85% (c)	(1.05%)	101%	$69,471
Institutional Class
Year Ended 12/31/2023	$10.93	21.85% (b)	0.90%	0.83% (c)	(0.21%)	64%	$2,110,299
Year Ended 12/31/2022	$8.97	(33.78%)	0.83% (d)	0.83% (c), (d)	(0.28%)	64%	$1,865,518
Year Ended 12/31/2021	$14.22	8.99%	0.83%	0.83% (c)	(0.55%)	75%	$3,413,006
Year Ended 12/31/2020	$16.60	29.51% (b)	0.86%	0.86% (c)	(0.30%)	86%	$3,569,794
Year Ended 12/31/2019	$14.70	26.60% (b)	0.86%	0.85% (c)	(0.05%)	101%	$3,117,486
Institutional 2 Class
Year Ended 12/31/2023	$12.06	21.94% (b)	0.87%	0.81%	(0.18%)	64%	$28,765
Year Ended 12/31/2022	$9.89	(33.83%) (b)	0.81% (d)	0.80% (d)	(0.26%)	64%	$23,445
Year Ended 12/31/2021	$15.62	9.09% (b)	0.80%	0.80%	(0.52%)	75%	$52,797
Year Ended 12/31/2020	$17.88	29.50% (b)	0.83%	0.82%	(0.27%)	86%	$61,000
Year Ended 12/31/2019	$15.69	26.63% (b)	0.83%	0.82%	(0.02%)	101%	$51,584
Institutional 3 Class
Year Ended 12/31/2023	$12.28	21.95% (b)	0.83%	0.76%	(0.17%)	64%	$28,181
Year Ended 12/31/2022	$10.07	(33.77%)	0.77% (d)	0.77% (d)	(0.23%)	64%	$47,945
Year Ended 12/31/2021	$15.88	9.16%	0.75%	0.75%	(0.46%)	75%	$78,955
Year Ended 12/31/2020	$18.12	29.54% (b)	0.78%	0.78%	(0.25%)	86%	$155,338
Year Ended 12/31/2019	$15.87	26.74% (b)	0.78%	0.77%	0.03%	101%	$65,408

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges

As noted in the Choosing a Share Class section of the prospectus, the sales charge reductions and waivers available to investors who purchase and hold their Fund shares through different financial intermediaries may vary. This Appendix A describes financial intermediary-specific reductions and/or waiver policies applicable to Fund shares purchased and held through the particular financial intermediary. A reduction and/or waiver that is specific to a particular financial intermediary is not available to Direct-at-Fund Accounts and does not apply in any case to non-omnibus positions wherever held. These reductions and/or waivers may apply to purchases, sales, and exchanges of Fund shares. A shareholder transacting in Fund shares through a financial intermediary identified below should carefully read the terms and conditions of the reductions and/or waivers. Please consult your financial intermediary with respect to any sales charge reduction/waiver described below.

The financial intermediary-specific information below may be provided by, or compiled from or based on information provided by, the financial intermediaries noted. While the Funds, the Investment Manager and the Distributor do not establish these financial intermediary-specific policies, our representatives are available to answer questions about these financial intermediary-specific policies and can direct you to the financial intermediary if you need help understanding them.

Ameriprise Financial Services, LLC (Ameriprise Financial Services)

The following information has been provided by Ameriprise Financial Services:

Class A Shares Front-End Sales Charge Waivers Available at Ameriprise Financial Services:

The following information applies to Class A shares purchases if you have an account with or otherwise purchase Fund shares through Ameriprise Financial Services:

Shareholders purchasing Fund shares through an Ameriprise Financial Services brokerage account are eligible for the following front-end sales charge waivers, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI:

■

Employer-sponsored retirement plans (e.g., 401(k) plans, 457 plans, employer-sponsored 403(b) plans, profit sharing and money purchase pension plans and defined benefit plans). For purposes of this provision, employer-sponsored retirement plans do not include SEP IRAs, Simple IRAs or SAR-SEPs.

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same fund (but not any other fund within the Columbia Fund family).

■

Shares exchanged from Class C shares of the same fund in the month of or following the 7-year anniversary of the purchase date. To the extent that the Fund’s Class C Shares – Conversion to Class A Shares policy (stated outside this Appendix A) provides for a waiver with respect to exchanges of Class C shares or the conversion of Class C shares following a shorter holding period, that waiver will apply.

■

Employees and registered representatives of Ameriprise Financial Services or its affiliates and their immediate family members.

■

Shares purchased by or through qualified accounts (including IRAs, Coverdell Education Savings Accounts, 401(k)s, 403(b) TSCAs subject to ERISA and defined benefit plans) that are held by a covered family member, defined as an Ameriprise financial advisor and/or the advisor’s spouse, advisor’s lineal ascendant (mother, father, grandmother, grandfather, great grandmother, great grandfather), advisor’s lineal descendant (son, step-son, daughter, step-daughter, grandson, granddaughter, great grandson, great granddaughter) or any spouse of a covered family member who is a lineal descendant.

■

Shares purchased from the proceeds of redemptions from another fund in the Columbia Fund family, provided (1) the repurchase occurs within 90 days following the redemption, (2) the redemption and purchase occur in the same account, and (3) redeemed shares were subject to a front-end or deferred sales load (i.e. Rights of Reinstatement).

A-1

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

Robert W. Baird & Co. Incorporated (Baird)

The following information has been provided by Baird:

Effective June 30, 2020, shareholders purchasing Columbia Fund shares through a Baird platform or account that maintains an omnibus position with the Fund will only be eligible for the following sales charge waivers (front-end sales charge waivers and CDSC waivers) and discounts, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI. A reduction and/or waiver that is specific to Baird will not apply to non-omnibus positions.

Front-End Sales Charge Waivers on Class A Shares Available at Baird:

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same Columbia Fund.

■

Share purchases by employees and registered representatives of Baird or its affiliates and their family members as designated by Baird.

■

Shares purchased with the proceeds of redemptions from another Columbia Fund, provided (1) the repurchase occurs within 90 days following the redemption, (2) the redemption and purchase occur in the same accounts, and (3) redeemed shares were subject to a front-end or deferred sales charge (known as rights of reinstatement).

■

A shareholder in the Fund’s Class C shares will have their shares converted at net asset value to Class A shares of the same Columbia Fund if the shares are no longer subject to CDSC and the conversion is in line with the policies and procedures of Baird.

■

Employer-sponsored retirement plans or charitable accounts in a transactional brokerage account at Baird, including 401(k) plans, 457 plans, employer-sponsored 403(b) plans, profit sharing and money purchase pension plans and defined benefit plans. For purposes of this provision, employer-sponsored retirement plans do not include SEP IRAs, Simple IRAs or SAR-SEPs.

CDSC Waivers on Class A and Class C Shares Available at Baird:

■

Shares sold due to death or disability of the shareholder.

■

Shares sold as part of a systematic withdrawal plan as described in this prospectus.

■

Shares purchased due to returns of excess contributions from an IRA account.

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts due to the shareholder reaching the qualified age based on applicable IRS regulations.

■

Shares sold to pay Baird fees but only if the transaction is initiated by Baird.

■

Shares acquired through a right of reinstatement.

Front-End Sales Charge Discounts Available at Baird: Breakpoints and/or Rights of Accumulations:

■

Breakpoints as described in this prospectus.

■

Rights of accumulations which entitle shareholders to breakpoint discounts will be automatically calculated based on the aggregated holding of Columbia Fund assets held by accounts within the purchaser’s household at Baird. Eligible Columbia Fund assets not held at Baird may be included in the rights of accumulations calculation only if the shareholder notifies his or her financial advisor about such assets.

■

Letters of Intent (LOI) allow for breakpoint discounts based on anticipated purchases of Columbia Funds through Baird, over a 13-month period of time.

Prospectus 2024

A-2

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

Edward D. Jones & Co., L.P. (Edward Jones)

Policies Regarding Transactions Through Edward Jones

The following information has been provided by Edward Jones:

Effective on or after January 1, 2024, the following information supersedes prior information with respect to transactions and positions held in Columbia Fund shares through an Edward Jones system. Clients of Edward Jones (also referred to as "shareholders") purchasing Columbia Fund shares on the Edward Jones commission and fee-based platforms are eligible only for the following sales charge discounts (also referred to as "breakpoints") and waivers, which can differ from discounts and waivers described elsewhere in this prospectus or the Fund’s SAI or through another broker-dealer. In all instances, it is the shareholder's responsibility to inform Edward Jones at the time of purchase of any relationship, holdings of Columbia Funds and Future Scholars Program, or other facts qualifying the purchaser for discounts or waivers. Edward Jones can ask for documentation of such circumstance. Shareholders should contact Edward Jones if they have questions regarding their eligibility for these discounts and waivers.

Breakpoints

■

Breakpoint pricing, otherwise known as volume pricing, at dollar thresholds as described in this prospectus.

Rights of Accumulation (ROA)

■

The applicable sales charge on a purchase of Class A shares is determined by taking into account all share classes (except certain money market funds and any assets held in group retirement plans) of Columbia Funds and Future Scholars Program held by the shareholder or in an account grouped by Edward Jones with other accounts for the purpose of providing certain pricing considerations ("pricing groups"). If grouping assets as a shareholder, this includes all share classes held on the Edward Jones platform and/or held on another platform. The inclusion of eligible Columbia Fund assets in the ROA calculation is dependent on the shareholder notifying Edward Jones of such assets at the time of calculation. Money market funds are included only if such shares were sold with a sales charge at the time of purchase or acquired in exchange for shares purchased with a sales charge.

■

The employer maintaining a SEP IRA plan and/or SIMPLE IRA plan may elect to establish or change ROA for the IRA accounts associated with the plan to a plan-level grouping as opposed to including all share classes at a shareholder or pricing group level.

■

ROA is determined by calculating the higher of cost minus redemptions or market value (current shares x NAV).

Letter of Intent (LOI)

■

Through a LOI, shareholders can receive the sales charge and breakpoint discounts for purchases shareholders intend to make over a 13-month period from the date Edward Jones receives the LOI. The LOI is determined by calculating the higher of cost or market value of qualifying holdings at LOI initiation in combination with the value that the shareholder intends to buy over a 13-month period to calculate the front-end sales charge and any breakpoint discounts. Each purchase the shareholder makes during that 13-month period will receive the sales charge and breakpoint discount that applies to the total amount. The inclusion of eligible Columbia Fund assets in the LOI calculation is dependent on the shareholder notifying Edward Jones of such assets at the time of calculation. Purchases made before the LOI is received by Edward Jones are not adjusted under the LOI and will not reduce the sales charge previously paid. Sales charges will be adjusted if LOI is not met.

■

If the employer maintaining a SEP IRA plan and/or SIMPLE IRA plan has elected to establish or change ROA for the IRA accounts associated with the plan to a plan-level grouping, LOIs will also be at the plan-level and may only be established by the employer.

A-3

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

Sales Charge Waivers

Sales charges are waived for the following shareholders and in the following situations:

■

Associates of Edward Jones and its affiliates and other accounts in the same pricing group (as determined by Edward Jones under its policies and procedures) as the associate. This waiver will continue for the remainder of the associate's life if the associate retires from Edward Jones in good-standing and remains in good standing pursuant to Edward Jones' policies and procedures.

■

Shares purchased in an Edward Jones fee-based program.

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment.

■

Shares purchased from the proceeds of redeemed shares of Columbia Funds so long as the following conditions are met: 1) the proceeds are from the sale of shares within 60 days of the purchase, and 2) the sale and purchase are made from a share class that charges a front-end load and one of the following:

■

The redemption and repurchase occur in the same account.

■

The redemption proceeds are used to process an: IRA contribution, excess contributions, conversion, recharacterizing of contributions, or distribution, and the repurchase is done in an account within the same Edward Jones grouping for ROA.

■

Shares exchanged into Class A shares from another share class so long as the exchange is into the same fund and was initiated at the discretion of Edward Jones. Edward Jones is responsible for any remaining CDSC due to the fund company, if applicable. Any future purchases are subject to the applicable sales charge as disclosed in this prospectus.

■

Exchanges from Class C shares to Class A shares of the same fund, generally, in the 84th month following the anniversary of the purchase date or earlier at the discretion of Edward Jones.

■

Purchases of Class 529-A shares through a rollover from either another education savings plan or a security used for qualified distributions.

■

Purchases of Class 529 shares made for recontribution of refunded amounts.

Contingent Deferred Sales Charge (CDSC) Waivers

If the shareholder purchases shares that are subject to a CDSC and those shares are redeemed before the CDSC is expired, the shareholder is responsible to pay the CDSC except in the following conditions:

■

The death or disability of the shareholder.

■

Systematic withdrawals with up to 10% per year of the account value.

■

Return of excess contributions from an Individual Retirement Account (IRA).

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts if the redemption is taken in or after the year the shareholder reaches qualified age based on applicable IRS regulations.

■

Shares sold to pay Edward Jones fees or costs in such cases where the transaction is initiated by Edward Jones.

■

Shares exchanged in an Edward Jones fee-based program.

■

Shares acquired through NAV reinstatement.

■

Shares redeemed at the discretion of Edward Jones for Minimum Balances, as described below.

Other Important Information Regarding Transactions Through Edward Jones

Minimum Purchase Amounts

■

Initial purchase minimum: $250

■

Subsequent purchase minimum: none

Prospectus 2024

A-4

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

Minimum Balances

■

Edward Jones has the right to redeem at its discretion Fund holdings with a balance of $250 or less. The following are examples of accounts that are not included in this policy:

■

A fee-based account held on an Edward Jones platform.

■

A 529 account held on an Edward Jones platform.

■

An account with an active systematic investment plan or LOI.

Exchanging Share Classes

■

At any time it deems necessary, Edward Jones has the authority to exchange at NAV a shareholder's holdings in the Fund to Class A shares.

Janney Montgomery Scott LLC (Janney)

The following information has been provided by Janney:

Effective May 1, 2020, if you purchase Columbia Fund shares through a Janney brokerage account, you will be eligible for the following load waivers (front-end sales charge waivers and CDSC, or back-end sales charge, waivers) and discounts, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI. A reduction and/or waiver that is specific to Janney does not apply to non-omnibus positions.

Front-End Sales Charge* Waivers on Class A Shares Available at Janney

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same fund (but not any other Columbia Fund).

■

Shares purchased by employees and registered representatives of Janney or its affiliates and their family members as designated by Janney.

■

Shares purchased from the proceeds of redemptions from another Columbia Fund, provided (1) the repurchase occurs within ninety (90) days following the redemption, (2) the redemption and purchase occur in the same account, and (3) redeemed shares were subject to a front-end or deferred sales load (i.e., right of reinstatement).

■

Shares acquired through a right of reinstatement.

■

Class C shares that are no longer subject to a CDSC and are converted to Class A shares of the same fund pursuant to Janney’s policies and procedures.

CDSC Waivers on Class A and C Shares Available at Janney

■

Shares sold upon the death or disability of the shareholder.

■

Shares sold as part of a systematic withdrawal plan as described in this prospectus.

■

Shares purchased in connection with a return of excess contributions from an IRA account.

■

Shares sold to pay Janney fees but only if the transaction is initiated by Janney.

■

Shares acquired through a right of reinstatement.

■

Shares exchanged into the same share class of a different fund.

Front-End Sales Charge* Discounts Available at Janney: Breakpoints, Rights of Accumulation, and/or Letters of Intent

■

Breakpoints as described in this prospectus.

A-5

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

■

Rights of accumulation (“ROA”), which entitle shareholders to breakpoint discounts, will be automatically calculated based on the aggregated holding of Columbia Fund assets held by accounts within the purchaser’s household at Janney. Eligible Columbia Fund assets not held at Janney may be included in the ROA calculation only if the shareholder notifies his or her financial advisor about such assets.

■

Letters of intent which allow for breakpoint discounts based on anticipated purchases within the Columbia Funds, over a 13-month time period. Eligible Columbia Fund assets not held at Janney may be included in the calculation of letters of intent only if the shareholder notifies his or her financial advisor about such assets.

* Also referred to as an “initial sales charge”.

J.P. Morgan Securities LLC

The following information has been provided by J.P. Morgan Securities LLC:

Effective September 29, 2023, if you purchase or hold fund shares through an applicable J.P. Morgan Securities LLC brokerage account, you will be eligible for the following sales charge waivers (front-end sales charge waivers and contingent deferred sales charge (“CDSC”), or back-end sales charge, waivers), share class conversion policy and discounts, which may differ from those disclosed elsewhere in this prospectus or SAI.

Front-end sales charge waivers on Class A shares available at J.P. Morgan Securities LLC

■

Shares exchanged from Class C (i.e. level-load) shares that are no longer subject to a CDSC and are exchanged into Class A shares of the same fund pursuant to J.P. Morgan Securities LLC’s share class exchange policy.

■

Qualified employer-sponsored defined contribution and defined benefit retirement plans, nonqualified deferred compensation plans, other employee benefit plans and trusts used to fund those plans. For purposes of this provision, such plans do not include SEP IRAs, SIMPLE IRAs, SAR-SEPs or 501(c)(3) accounts.

■

Shares of funds purchased through J.P. Morgan Securities LLC Self-Directed Investing accounts.

■

Shares purchased through rights of reinstatement.

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same fund (but not any other fund within the Columbia fund family).

■

Shares purchased by employees and registered representatives of J.P. Morgan Securities LLC or its affiliates and their spouse or financial dependent as defined by J.P. Morgan Securities LLC.

Class C to Class A share conversion

■

A shareholder in the fund’s Class C shares will have their shares converted to Class A shares (or the appropriate share class) of the same fund if the shares are no longer subject to a CDSC and the conversion is consistent with J.P. Morgan Securities LLC’s policies and procedures.

CDSC waivers on Class A and C shares available at J.P. Morgan Securities LLC

■

Shares sold upon the death or disability of the shareholder.

■

Shares sold as part of a systematic withdrawal plan as described in this prospectus.

■

Shares purchased in connection with a return of excess contributions from an IRA account.

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts pursuant to the Internal Revenue Code.

■

Shares acquired through a right of reinstatement.

Front-end load discounts available at J.P. Morgan Securities LLC: breakpoints, rights of accumulation & letters of intent

■

Breakpoints as described in this prospectus.

Prospectus 2024

A-6

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

■

Rights of Accumulation (“ROA”) which entitle shareholders to breakpoint discounts as described in this prospectus will be automatically calculated based on the aggregated holding of Columbia fund family assets held by accounts within the purchaser’s household at J.P. Morgan Securities LLC. Eligible Columbia fund family assets not held at J.P. Morgan Securities LLC (including 529 program holdings, where applicable) may be included in the ROA calculation only if the shareholder notifies their financial advisor about such assets.

■

Letters of Intent (“LOI”) which allow for breakpoint discounts based on anticipated purchases within the Columbia fund family, through J.P. Morgan Securities LLC, over a 13-month period of time (if applicable).

Merrill Lynch Pierce, Fenner & Smith Incorporated (Merrill)

The following information has been provided by Merrill:

Purchases or sales of front-end (i.e. Class A) or level-load (i.e., Class C) mutual fund shares through a Merrill platform or account will be eligible only for the following sales load waivers (front-end, contingent deferred, or back-end waivers) and discounts, which differ from those disclosed elsewhere in this Fund’s prospectus. Purchasers will have to buy mutual fund shares directly from the mutual fund company or through another intermediary to be eligible for waivers or discounts not listed below.

It is the client’s responsibility to notify Merrill at the time of purchase or sale of any relationship or other facts that qualify the transaction for a waiver or discount. A Merrill representative may ask for reasonable documentation of such facts and Merrill may condition the granting of a waiver or discount on the timely receipt of such documentation.

Additional information on waivers and discounts is available in the Merrill Sales Load Waiver and Discounts Supplement (the “Merrill SLWD Supplement") and in the Mutual Fund Investing at Merrill pamphlet at ml.com/funds. Clients are encouraged to review these documents and speak with their financial advisor to determine whether a transaction is eligible for a waiver or discount.

Front-End Load Waivers Available at Merrill

■

Shares of mutual funds available for purchase by employer-sponsored retirement, deferred compensation, and employee benefit plans (including health savings accounts) and trusts used to fund those plans, provided the shares are not held in a commission-based brokerage account and shares are held for the benefit of the plan. For purposes of this provision, employer-sponsored retirement plans do not include SEP IRAs, Simple IRAs, SAR-SEPs or Keogh plans.

■

Shares purchased through a Merrill investment advisory program.

■

Brokerage class shares exchanged from advisory class shares due to the holdings moving from a Merrill investment advisory program to a Merrill brokerage account.

■

Shares purchased through the Merrill Edge Self-Directed platform.

■

Shares purchased through the systematic reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same mutual fund in the same account.

■

Shares exchanged from level-load shares to front-end load shares of the same mutual fund in accordance with the description in the Merrill SLWD Supplement.

■

Shares purchased by eligible employees of Merrill or its affiliates and their family members who purchase shares in accounts within the employee’s Merrill Household (as defined in the Merrill SLWD Supplement).

■

Shares purchased by eligible persons associated with the Fund as defined in this prospectus (e.g., the Fund’s officers or trustees).

■

Shares purchased from the proceeds of a mutual fund redemption in front-end load shares provided (1) the repurchase is in a mutual fund within the same fund family, (2) the repurchase occurs within 90 calendar days

A-7

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

from the redemption trade date, and (3) the redemption and purchase occur in the same account (known as Rights of Reinstatement). Automated transactions (i.e. systematic purchases and withdrawals) and purchases made after shares are automatically sold to pay Merrill’s account maintenance fees are not eligible for Rights of Reinstatement.

Contingent Deferred Sales Charge (CDSC) Waivers on Front-end, Back-end, and Level Load Shares Available at Merrill

■

Shares sold due to the client’s death or disability (as defined by Internal Revenue Code Section 22e(3)).

■

Shares sold pursuant to a systematic withdrawal program subject to Merrill’s maximum systematic withdrawal limits as described in the Merrill SLWD Supplement.

■

Shares sold due to return of excess contributions from an IRA account.

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts due to the investor reaching the qualified age based on applicable IRS regulation.

■

Front-end or level-load shares held in commission-based, non-taxable requirement brokerage accounts (e.g., traditional, Roth, rollover, SEP IRAs, Simple IRAs, SAR-SEPs or Keogh plans) that are transferred to fee-based accounts or platforms and exchanged for a lower cost share class of the same mutual fund.

Front-end Load Discounts Available at Merrill: Breakpoints, Rights of Accumulation & Letters of Intent

■

Breakpoints discounts, as described in this prospectus, where the sales load is at or below the maximum sales load that Merrill permits to be assessed to a front-end load purchase, as described in the Merrill SLWD Supplement.

■

Rights of Accumulation (ROA), as described in the Merrill SLWD Supplement, which entitle clients to breakpoint discounts based on the aggregated holding of mutual fund family assets held in accounts in their Merrill Household.

■

Letters of Intent (LOI), which allow for breakpoint discounts on eligible new purchases based on anticipated future eligible purchases within a fund family at Merrill, in accounts within your Merrill Household, as further described in the Merrill SLWD Supplement.

Morgan Stanley Smith Barney, LLC (Morgan Stanley Wealth Management)

The following information has been provided by Morgan Stanley Wealth Management:

Effective July 1, 2018, shareholders purchasing Columbia Fund shares through a Morgan Stanley Wealth Management transactional brokerage account will be eligible only for the following front-end sales charge waivers with respect to Class A shares, which may differ from and may be more limited than those disclosed elsewhere in this prospectus or the Fund’s SAI.

Front-End Sales Charge Waivers on Class A Shares Available at Morgan Stanley Wealth Management Effective July 1, 2018:

■

Morgan Stanley employee and employee-related accounts according to Morgan Stanley’s account linking rules.

■

Shares purchased through reinvestment of dividends and capital gains distributions when purchasing shares of the same fund.

■

Shares purchased through a Morgan Stanley self-directed brokerage account.

■

Class C (i.e., level-load) shares that are no longer subject to a CDSC and are exchanged for Class A shares of the same fund pursuant to Morgan Stanley Wealth Management’s share class exchange program.

Prospectus 2024

A-8

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

■

Shares purchased from the proceeds of redemptions from another Columbia Fund, provided (i) the repurchase occurs within 90 days following the redemption, (ii) the redemption and purchase occur in the same account, and (iii) redeemed shares were subject to a front-end or deferred sales charge.

Raymond James & Associates, Inc., Raymond James Financial Services, Inc. and each entity’s affiliates (Raymond James)

The following information has been provided by Raymond James:

Intermediary-Defined Sales Charge Waiver Policies:

The availability of certain initial or deferred sales charge waivers and discounts may depend on the particular financial intermediary or type of account through which you purchase or hold Columbia Fund shares.

Intermediaries may have different policies and procedures regarding the availability of front-end sales load waivers or contingent deferred (back-end) sales load (CDSC) waivers, which are discussed below. In all instances, it is the purchaser’s responsibility to notify the Fund or the purchaser’s financial intermediary at the time of purchase of any relationship or other facts qualifying the purchaser for sales charge waivers or discounts. For waivers and discounts not available through a particular intermediary, shareholders will have to purchase Columbia Fund shares directly from the Fund or through another intermediary to receive these waivers or discounts.

Raymond James:

Effective March 1, 2019, shareholders purchasing Columbia Fund shares through a Raymond James platform or account, or through an introducing broker-dealer or independent registered investment adviser for which Raymond James provides trade execution, clearance, and/or custody services, will be eligible only for the following load waivers (front-end sales charge waivers and contingent deferred, or back-end, sales charge waivers) and discounts, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI.

Front-End Sales Load Waivers on Class A Shares Available at Raymond James:

■

Shares purchased in an investment advisory program.

■

Shares purchased within the Columbia Funds through a systematic reinvestment of capital gains and dividend distributions.

■

Employees and registered representatives of Raymond James or its affiliates and their family members as designated by Raymond James.

■

Shares purchased from the proceeds of redemptions within the Columbia Funds, provided (1) the repurchase occurs within 90 days following the redemption, (2) the redemption and purchase occur in the same account, and (3) redeemed shares were subject to a front-end or deferred sales load (known as Rights of Reinstatement).

■

A shareholder in the Fund’s Class C shares will have their shares converted at net asset value to Class A shares (or the appropriate share class) of the Fund if the shares are no longer subject to a CDSC and the conversion is in line with the policies and procedures of Raymond James.

CDSC Waivers on Class A and Class C Shares Available at Raymond James:

■

Death or disability of the shareholder.

■

Shares sold as part of a systematic withdrawal plan as described in this prospectus.

■

Return of excess contributions from an IRA Account.

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts due to the shareholder reaching the qualified age based on applicable IRS regulations as described in this prospectus.

■

Shares sold to pay Raymond James fees but only if the transaction is initiated by Raymond James.

■

Shares acquired through a right of reinstatement.

A-9

Prospectus 2024

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

Front-End Load Discounts Available at Raymond James: Breakpoints, Rights of Accumulation and/or Letters of Intent:

■

Breakpoints as described in this prospectus.

■

Rights of accumulation which entitle shareholders to breakpoint discounts will be automatically calculated based on the aggregated holding of Columbia Fund assets held by accounts within the purchaser’s household at Raymond James. Eligible Columbia Fund assets not held at Raymond James may be included in the calculation of rights of accumulation only if the shareholder notifies his or her financial advisor about such assets.

■

Letters of intent which allow for breakpoint discounts based on anticipated purchases within the Columbia Funds, over a 13-month time period. Eligible Columbia Fund assets not held at Raymond James may be included in the calculation of letters of intent only if the shareholder notifies his or her financial advisor about such assets.

Stifel, Nicolaus & Company, Incorporated (Stifel) and its broker dealer affiliates

The following information has been provided by Stifel:

Effective January 2, 2024, shareholders purchasing or holding Columbia Fund shares, including existing fund shareholders, through a Stifel or affiliated platform that provides trade execution, clearance, and/or custody services, will be eligible for the following sales charge load waivers (including front-end sales charge waivers and contingent deferred, or back-end, (CDSC) sales charge waivers) and discounts, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI.

CLASS A SHARES

As described elsewhere in this prospectus, Stifel may receive compensation out of the front-end sales charge if you purchase Class A shares through Stifel.

Rights of accumulation

■

Rights of accumulation (ROA) that entitle shareholders to breakpoint discounts on front-end sales charges will be calculated by Stifel based on the aggregated holding of all assets in all classes of shares of Columbia Funds held by accounts within the purchaser’s household at Stifel. Eligible Columbia Fund assets not held at Stifel may be included in the calculation of ROA only if the shareholder notifies his or her financial advisor about such assets.

■

Front-end sales charge waivers on Class A shares available at Stifel

Sales charges may be waived for the following shareholders and in the following situations:

■

Class C shares that have been held for more than seven (7) years may be converted to Class A shares or other front-end share class(es) of the same fund pursuant to Stifel's policies and procedures. To the extent that this prospectus elsewhere provides for a waiver with respect to the exchange or conversion of such shares following a shorter holding period, those provisions shall continue to apply.

■

Shares purchased by employees and registered representatives of Stifel or its affiliates and their family members as designated by Stifel.

■

Shares purchased in an Stifel fee-based advisory program, often referred to as a “wrap” program.

■

Shares purchased through reinvestment of capital gains distributions and dividend reinvestment when purchasing shares of the same or other fund within the Columbia Funds.

■

Shares purchased from the proceeds of redeemed shares of Columbia Funds so long as the proceeds are from the sale of shares from an account with the same owner/beneficiary within 90 days of the purchase. For the absence of doubt, shares redeemed through a Systematic Withdrawal Plan are not eligible for rights of reinstatement.

Prospectus 2024

A-10

Columbia Acorn® Fund

Appendix A: Financial Intermediary-Specific Reductions/Waivers of Sales Charges (continued)

■

Shares from rollovers into Stifel from retirement plans to IRAs.

■

Shares exchanged into Class A shares from another share class so long as the exchange is into the same fund and was initiated at the direction of Stifel. Stifel is responsible for any remaining CDSC due to the fund company, if applicable. Any future purchases are subject to the applicable sales charge as disclosed in this prospectus.

■

Purchases of Class 529-A shares through a rollover from another 529 plan.

■

Purchases of Class 529-A shares made for reinvestment of refunded amounts.

■

All other sales charge waivers and reductions described elsewhere in this prospectus or the Fund’s SAI still apply.

Contingent Deferred Sales Charges Waivers on Class A and C Shares

■

Death or disability of the shareholder or, in the case of 529 plans, the account beneficiary.

■

Shares sold as part of a systematic withdrawal plan not to exceed 12% annually.

■

Return of excess contributions from an IRA Account.

■

Shares sold as part of a required minimum distribution for IRA and retirement accounts due to the shareholder reaching the qualified age based on applicable IRS regulations.

■

Shares acquired through a right of reinstatement.

■

Shares sold to pay Stifel fees or costs in such cases where the transaction is initiated by Stifel.

■

Shares exchanged or sold in a Stifel fee-based program.

■

All other sales charge waivers and reductions described elsewhere in this prospectus or the Fund’s SAI still apply.

Share Class Conversions in Advisory Accounts

■

Stifel continually looks to provide our clients with the lowest cost share class available based on account type. Stifel reserves the right to convert shares to the lowest cost share class available at Stifel upon transfer of shares into an advisory program.

U.S. Bancorp Investments, Inc. (USBI)

The following information has been provided by USBI:

Effective September of 2021, shareholders purchasing Columbia Fund shares through a USBI platform or who own shares for which USBI is the broker-dealer, where the shares are held in an omnibus account, will only be eligible for the following front-end sales charge waivers and discounts, which may differ from those disclosed elsewhere in this prospectus or the Fund’s SAI.

All other sales charge waivers and reduction described elsewhere in this prospectus or the Fund’s SAI still apply.

USBI Conversion of Class C shares

Class C (i.e., level-load) shares that are no longer subject to a contingent deferred sales charge are systematically converted to the Class A shares of the same fund pursuant to USBI’s share class exchange policy.

Additional Sales Charge Reductions and/or Waivers Available at Certain Financial Intermediaries

Shareholders purchasing Columbia Fund shares through a platform or account of RBC Capital Markets, LLC are eligible for the following sales charge waiver:

A-11

Prospectus 2024

Columbia Acorn® Fund

P.O. Box 219104
Kansas City, MO 64121-9104

Columbia Acorn Family of Funds

Additional Information About the Fund

Additional information about the Fund’s investments is available in the Fund’s annual and semiannual reports to shareholders. In the annual report, you will find a discussion of the market conditions and investment strategies that significantly affected the Fund’s performance during its last fiscal year. The SAI also provides additional information about the Fund and its policies. The SAI, which has been filed with the SEC, is legally part of this prospectus (incorporated by reference). To obtain these documents free of charge, to request other information about the Fund and to make shareholder inquiries, please contact the Fund as follows:

By Mail: Columbia Management Investment Services Corp.
P.O. Box 219104
Kansas City, MO 64121-9104

By Telephone: 800.345.6611

Online: columbiathreadneedleus.com

Shareholder Communications with the Board

The Fund’s Board of Trustees has adopted procedures by which shareholders may communicate with the Board. Shareholders who wish to communicate with the Board should send their written communications to the Board by mail, c/o Columbia Wanger Asset Management, LLC, 71 S. Wacker Drive, Suite 2500, Chicago, IL 60606, Attention: Secretary. Shareholder communications must (i) be in writing, (ii) identify the Columbia Acorn Fund to which the communication relates and (iii) state the particular class of shares and number of shares held by the communicating shareholder.

Obtaining Additional Information from the SEC

Reports and other information about the Fund are also available in the EDGAR Database on the SEC’s website at http://www.sec.gov. You can receive copies of this information, for a duplication fee, by electronic request at the following e-mail address: [email protected].

The investment company registration number of Columbia Acorn Trust, of which the Fund is a series, is 811-01829.

Columbia Threadneedle Investments is the global brand name of the Columbia and Threadneedle group of companies.
The Fund is distributed by Columbia Management Investment Distributors, Inc., 290 Congress Street, Boston, MA 02210.

PRO110_12_P01_(05/24)

Class	Ticker Symbol
A	LACAX
Advisor (Class Adv)	CEARX
C	LIACX
Institutional (Class Inst)	ACRNX

Class	Ticker Symbol
Institutional 2 (Class Inst2)	CRBRX
Institutional 3 (Class Inst3)	CRBYX
S (a)	—