SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 6-K

REPORT OF FOREIGN PRIVATE ISSUER

PURSUANT TO RULE 13a-16 OR 15d-16 UNDER

THE SECURITIES EXCHANGE ACT OF 1934

For the month of August 2016

Commission File Number: 000-53445

KB Financial Group Inc.

(Translation of registrant’s name into English)

84, Namdaemoon-ro, Jung-gu, Seoul, 04534, Korea

(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):  ¨

Note: Regulation S-T Rule 101(b)(1) only permits the submission in paper of a Form 6-K if submitted solely to provide an attached annual report to security holders.

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):  ¨

Note: Regulation S-T Rule 101(b)(7) only permits the submission in paper of a Form 6-K if submitted to furnish a report or other document that the registrant foreign private issuer must furnish and make public under the laws of the jurisdiction in which the registrant is incorporated, domiciled or legally organized (the registrant’s “home country”), or under the rules of the home country exchange on which the registrant’s securities are traded, as long as the report or other document is not a press release, is not required to be and has not been distributed to the registrant’s security holders, and, if discussing a material event, has already been the subject of a Form 6-K submission or other Commission filing on EDGAR.

Report of Material Event

On August 2, 2016, the board of directors of KB Financial Group Inc. (“KB Financial Group”) resolved to effect a comprehensive stock swap (the “Stock Swap”) pursuant to which the common shares of Hyundai Securities Co., Ltd. (“Hyundai Securities”) would be exchanged for common shares of KB Financial Group, and entered into a stock swap agreement (the “Stock Swap Agreement”) with Hyundai Securities in connection therewith. The details of the Stock Swap are as follows:

“Small-scale stock swap” pursuant to Article 360-10 of the Korean Commercial Code (the “KCC”)

KB Financial Group: Hyundai Securities = 1: 0.1907312

As KB Financial Group and Hyundai Securities are both “stock-listed corporations,” the swap ratio for the Stock Swap was determined based on a swap price calculated in accordance with Article 165-4 of the Financial Investment Services and Capital Markets Act (the “FSCMA”) and paragraph (1) of Article 176-5 and paragraph (2) of Article 176-6 of the Enforcement Decree of the FSCMA.

With the date preceding the earlier of the date of the resolution of the board of directors for the Stock Swap (August 2, 2016) and the date of the Stock Swap Agreement (August 2, 2016) as the calculation date (August 1, 2016), the swap price was calculated as the average of (x) the volume weighted average of the closing prices of the applicable common shares for the latest one-month period, (y) the volume weighted average of the closing prices of the applicable common shares for the latest one-week period and (z) the closing price of the applicable common shares as of the latest date.

In addition, paragraph (1)-1 of Article 176-5 of the Enforcement Decree of the FSCMA allows for the application of a discount or premium of up to 30% (10% in case of mergers between affiliates) of the swap price calculated in accordance with the above method. However, no such discount or premium was applied in calculating the swap price for the Stock Swap.

Through the addition of Hyundai Securities as a wholly-owned subsidiary, KB Financial Group seeks to achieve synergies by increasing management efficiency and strengthening the unity of KB Financial Group, thereby enhancing its corporate value.

The Stock Swap is expected to have a dilutive effect on the share ownership percentages of the existing shareholders of KB Financial Group in accordance with the swap ratio. However, there will be no change in control for corporate governance purposes, and KB Financial Group and Hyundai Securities will continue to exist as separate entities.

In addition, pursuant to the Stock Swap Agreement, the terms of office of the directors and audit committee members of KB Financial Group who were appointed prior to the Stock Swap will remain the same following the Stock Swap despite Article 360-13 of the KCC. No new directors will be appointed as a result of the Stock Swap.

KB Financial Group expects that the Stock Swap will result in the improvement of certain of its group-level performance metrics (including its return on equity, double leverage ratio and debt ratio) and an increase in its capital through the issuance of new common shares.

Although the assets and liabilities of Hyundai Securities will remain unchanged and only its shareholder composition will change, Hyundai Securities expects that the Stock Swap will lead to a reduction of its funding costs, due to an improvement in its credit ratings that may result from its becoming a wholly-owned subsidiary of KB Financial Group.

The Stock Swap is expected to promote sustainable growth by contributing to the maximization of synergies among affiliated companies of KB Financial Group, through an increase in customer convenience and benefits and the strengthening of its competitiveness.

The Stock Swap will proceed as a small-scale stock swap pursuant to Article 360-10 of the KCC. Accordingly, shareholders of KB Financial Group will not be granted any appraisal rights.

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.