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FORM 6-K

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Report of Foreign Issuer

Pursuant to Rule 13a-16 or 15d-16 of

the Securities Exchange Act of 1934

For the month of April, 2015

Commission File Number: 001-12518

Banco Santander, S.A.

(Exact name of registrant as specified in its charter)

Ciudad Grupo Santander

28660 Boadilla del Monte (Madrid) Spain

(Address of principal executive office)

Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F:

Form 20-F  x            Form 40-F  ¨

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1):

Yes  ¨            No   x

Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7):

Yes  ¨            No   x

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Banco Santander, S.A.

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INCORPORATION BY REFERENCE

Items 2 and 3 of this report on Form 6-K shall be deemed to be incorporated by reference into the registration statement on Form F-3 (Registration Number: 333-185153) of Banco Santander, S.A. and to be a part thereof from the date on which this report is filed, to the extent not superseded by documents or reports subsequently filed or furnished.

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Item 1

PRESS RELEASE

FIRST QUARTER 2015 RESULTS

Banco Santander made a profit EUR 1.717

billion, 32% more than a year earlier

Madrid, April 28, 2015. Banco Santander made attributable profit of EUR 1,717 million in the first quarter, 32% more than the same period of 2014. Of the ten core markets in which the group operates, only Chile showed a decline in profit. The strong growth was supported by the bank’s three largest markets, Spain, UK and Brazil.

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Results

Exchange rates, marked by the euro-dollar and euro-pound depreciation, had a positive impact on growth rates in the balance sheet and results for the first quarter of 2015, adding to the favourable trend in the business and in group revenues. Attributable profit for the quarter grew 32%, a change that would have been 22% without the exchange rate effect. Revenues increased 13%, 7% without the exchange rate factor; and lending grew 14%, 7% without the exchange rate variation.

These results are produced in a context of uneven economic performance for the different countries in which the group operates. Countries such as U.S., Mexico and Poland are expected to grow more than 3% this year; Spain, U.K. and Chile, more than 2.5%; and Germany and Portugal, over 1.5%. Meanwhile, Brazil and Argentina will register a fall in economic growth. Official interest rates in euros, dollars and pounds are at historic lows.

The improvement in profits are a result of an increase in basic revenue of 13%, a change that is two points greater than costs, which grew 11%. This performance allows continued improvement in the efficiency ratio of almost one point, to 47.0%, which positions Banco Santander among the most efficient financial institutions in the world.

Net operating income, the difference between income and costs, stood at EUR 6,067 million, with growth of 15%. This increase, together with the decrease in loan loss provisions of 5%, means that the final result shows a 32% improvement.

Emerging economies (Latin America and Poland) accounted for 42% of profit and mature markets contributed 58%. By country, the largest contribution was from Brazil (21%), followed by U.K. (20%), Spain (15%), U.S. (10%), Mexico (7%), Chile (5%), Poland (4%), Argentina (3%) and Portugal (2%). Santander Consumer Finance contributed 10% to total profit.

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Balance sheet

Banco Santander closed March 2015 with total assets of EUR 1.37 trillion, 17% more than March of 2014. The business shows balanced growth, since both new loans and deposits grew 14%.

At the end of March, 2015, the loan portfolio reached EUR 813,260 million, 14% more than the year before (7% without the exchange rate effect), which means the credit balances increased by more than EUR 100,000 in one year. Of the ten key markets in which the group operates, credit grew in all except Portugal.

In Spain, the total loan portfolio reached EUR 161,023 million, with an increase slightly above 1% compared to the close of March, 2014, after subtracting repos. New lending to companies grew 24% and to individuals, 36%, with mortgages increasing 23%. These increases in new credit exceed redemptions generating growth in final balances of financing to companies, consumer lending to individuals and to the public administration. However, in mortgages, amounts redeemed exceeded new transactions.

In U.K., the loan portfolio stood at EUR 275,654 million, with growth of 20% over the first quarter of 2014. Without the exchange rate effect, in pounds, the increase in lending is 5%, boosted by financing to companies, which grew 9% year-on-year, mortgages, which grew 1%, and the addition of GBP 2,500 million in automobile loans as a result of the agreement with the French group PSA.

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In Brazil, the loan portfolio reached EUR 76,336 million, with growth of 5%, which is 17% without the exchange rate effect. Financing to companies was responsible for most of the growth, 39%, as well as in mortgages, which increased 35%. Financing to SMEs also showed marked improvement, and grew 6% year-on-year.

As for customer funds, total deposits and mutual funds of Grupo Santander is 780,020 million, with growth of 14% (8% without the exchange rate effect), which represents an addition of EUR 96,000 million in one year. Deposits reached 644,766 million and mutual funds 135,254 million, with growth of 11% and 21%, respectively, for the year. All markets gained resources.

Current accounts, which make up more than half of these deposits, showed positive growth in the group’s ten key markets. The current account balance reached EUR 393,588 million at the close of the first quarter of the year, with growth of 28% compared with the end of March, 2014. This means that in twelve months 85,000 million was added to these accounts. The current account is key to the strategy of increasing the loyalty of the bank’s customers, since it is the conduit for most transactions, centralizing income and costs.

In Spain, total customer funds (deposits plus mutual funds) amounted to 229,377 million at the end of March of this year, with an increase of 7% for the year. Demand deposits grew 18%, compared to a fall of 16% for time deposits, and assets in mutual funds rose 28%.

In U.K., the volume of deposits plus mutual funds reached EUR 223,932 million and grew 17% for the year, 3% without the exchange rate effect. The improvement is centred on growth in current accounts to individuals, the balance of which increased 40% in twelve months, with a rate of increase of one billion pounds per month. The 1|2|3 products have attracted 4 million customers, 1.3 million in the last year and for 94% of them, Santander is their primary bank.

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In Brazil, the total of deposits and mutual funds reached 85,802 million, with growth of 12% without the exchange rate effect. Total deposits grew 7% and funds, 17%.

Spain, U.K. and Brazil represented 69% of customer funds managed and commercialised by the group and 63% of the loan portfolio. Loan portfolio quality improved in the three countries, which determines the group’s NPL performance, which was at 4.85% at the close of the first quarter, with a decrease of 0.67 point in one year, of which more than half (0.34 point) occurred in the quarter. Meanwhile, the group continued to increase coverage, which is now at 69% compared to 66% a year earlier.

The NPL ratio in Spain is at 7.25%, 0.13 points less than last December and 0.36 points lower than one year ago. In U.K., the NPL ratio is very low, 1.75%, and even so decreased 0.13 point in one year. Brazil closed with a NPL ratio of 4.90% and managed to cut NPLs 0.15 point for the quarter and 0.84 points for the year.

Capital

At the beginning of the year the bank carried out a rights issue for EUR 7,500 million via an accelerated bookbuilding offer among institutional investors. At the close of the first quarter, Banco Santander’s total equity stood at EUR 91,915 million, 27% more than one year before. The bank’s capital ratio is 12%, with core capital of 9.7%, both fully loaded.

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The 32% growth in profit meant that despite an increase of 22% in the number of shares as a result of the rights issue and the scrip dividend, earnings per share grew 6%. Likewise, the return on tangible equity (ROTE) increased 1.1 points, to 11.5%.

Along with the rights issues, the bank’s board of directors announced its new dividend policy against fiscal year 2015, which consists of the payment of four dividends for EUR 0.05 per share, of which three will be paid in cash and one in shares or cash, per the shareholders choice. This year’s August dividend payment will be made entirely in cash.

Banco Santander has a market capitalization of more than EUR 90,000 million, making it the first bank in the Euro zone and eleventh-largest in the world. Santander has 3,230,808 shareholders and 187,262 employees serving over 100 million customers through 12,920 branches.

More information at: www.santander.com

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 KEY CONSOLIDATED DATA 

(*).- Variations w/o exchange rate

Quarterly: Net interest income: +0.9%; Gross income: +0.7%; Pre-provision profit:+1.0%; Attributable profit:+12.1%

Year-on-year: Net interest income: +7.9%; Gross income: +6.8%; Pre-provision profit: +8.1%; Attributable profit: +21.8%

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Item 2

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Financial report

At Banco Santander, we take advantage of new

communication technologies and the social

networks to improve dialogue with our

stakeholders

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KEY CONSOLIDATED DATA

(*).- Variations w/o exchange rate

Quarterly: Net interest income: +0.9%; Gross income: +0.7%; Pre-provision profit:+1.0%; Attributable profit:+12.1%

Year-on-year: Net interest income: +7.9%; Gross income: +6.8%; Pre-provision profit: +8.1%; Attributable profit: +21.8%

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Simple | Personal | Fair

The Human Resources Strategic Plan is being reviewed to ensure it meets the needs of the new working culture and contributes to the Group’s different strategic lines.

Of note among the initiatives taken during the quarter were:

We continued to transform our commercial banking model and make it more personalised and modern.

Several initiatives were launched during the quarter to increase customer satisfaction and loyalty:

In addition, the Bank continued to advance in strengthening multi-channels with new and better commercial websites, new applications and capabilities for mobile phones (Santander Wallet in Spain, app for tablet), as well as programmes to encourage clients to join digital channels in Spain, UK, Brazil and Portugal.

Several steps were taken during the quarter to enhance transparency with our shareholders and make it easier for them to exercise their rights. Particularly noteworthy was the 53% increase in meetings with shareholders, the implementation of an app to facilitate participation in General Shareholders’ Meetings by electronic devices (which multiplied by a factor of 7) and redesigning the commercial web Yo Soy Accionista. Recent data shows that 94% of shareholders that use our off-site assistance services would recommend it.

Grupo Santander continued to strengthen its commitment to society, with the following notable initiatives during the quarter:

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General background

Grupo Santander conducted its business in the first quarter in a more favourable economic environment in developed countries and also in many of the emerging markets where it operates. The expansion remained firm in the US and UK and in the euro zone the recovery began to consolidate, particularly in Germany and Spain. Among the emerging economies growth in Brazil, hit by adjustment policies, drought and some waning of confidence, slowed. Mexico and Chile, on the other hand, picked up and Poland kept up a satisfactory growth rate.

United States: growth moderated in the first quarter, although the trend remains close to 3%. Inflation, affected by the fall in oil prices, was around 0%, but the Federal Reserve, more attentive to the situation on the ground (underlying inflation of 1.3%) is preparing the market for a rise in interest rates that could begin in the summer.

Latin American economies performed to varying degrees:

Euro zone growth accelerated in the fourth quarter of 2014 to 0.3% quarter-on-quarter (0.9% year-on-year). The figures at the start of 2015 confirm the trend of a gradual upswing, although varying by country. Inflation has been negative since December, exerting downward pressure on inflation expectations. This led the ECB to approve a bond-buying programme between March 2015 and September 2016, which is favouring a softening of financial conditions and brought the euro to $1.08/€1.

United Kingdom: growth still robust, due the impact of lower oil prices on consumption, an increase in household incomes, inflation at 0% and sterling’s appreciation. The evolution of inflation once again cooled expectations of interest rate rises in the markets in the middle of 2016.

Poland: GDP in the fourth quarter rose 0.7% quarter-on-quarter (3.1% year-on-year). Economic fundamentals remain solid. Inflation reached a floor in February (-1.6%) and in March began to rise. The central bank lowered interest rates (key rate at 1.5%) and the cycle of cuts is estimated to have ended.

Exchange rates: 1 euro / currency parity

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u Grupo Santander results

First quarter highlights

Income statement EUR million

(*).- Stockholders’ equity: Shareholders’ equity + equity adjustments by valuation. In 2014, pro-forma taking into account the January 2015 capital increase.

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Gross income

Of note was Brazil which, for the second straight quarter rose and its year-on-year growth rate is already positive. Net interest income also increased in the UK, the US (due to the good performance of SCUSA), Santander Consumer Finance (favoured by the acquisitions and agreements), Spain and Mexico.

Chile’s net interest income, on the other hand, fell because of lower inflation in the quarter and Poland, due to the fall in interest rates.