UNITED STATES

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 February 2015

Commission File Number: 000-53445

KB Financial Group Inc.

(Translation of registrant’s name into English)

84, Namdaemoon-ro, Jung-gu, Seoul 100-703, 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):  ¨

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):  ¨

Indicate by check mark whether the registrant by furnishing the information contained in this Form is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.

Yes  ¨            No   x

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b): 82-N/A.

Preliminary Operating Results for Fiscal Year 2014

On February 5, 2015, KB Financial Group Inc. (“KB Financial Group”) held an earnings conference and released its preliminary operating results for fiscal year 2014. The following tables reflect the key figures released during the conference. The presentation materials for the conference, which contain further details, are available at KB Financial Group’s website (http://www.kbfg.com).

The preliminary figures presented herein are based on the International Financial Reporting Standards as adopted by the Republic of Korea (“K-IFRS”) and are currently being audited by our independent auditors and are subject to change.

Declaration of Cash Dividends by KB Financial Group Inc.

On February 5, 2015, the board of directors of KB Financial Group passed a resolution to declare a cash dividend of KRW 780 per common share (total dividend amount: KRW 301,354,320,540), subject to shareholder approval. The record date is December 31, 2014, and in accordance with the Korean Commercial Code, the payment for such dividends (which is subject to change pending shareholder approval) is expected to be made within one month following shareholder approval at the annual general meeting of shareholders of KB Financial Group, which is expected to be held on March 27, 2015.

The contents of the above declaration remain subject to change pending approval at the annual general meeting of shareholders and the results of the audit by our independent auditors.

The total number of common shares that are eligible for payment of dividends is 386,351,693 shares.

Report of Change in Revenue or Profit of More than 15% (Kookmin Bank)

On February 5, 2015, KB Financial Group reported changes in the revenue and profit of Kookmin Bank, a wholly-owned subsidiary of KB Financial Group, each on a consolidated basis, as described below.

Key Details

Declaration of Cash Dividends by Kookmin Bank

On February 5, 2015, the board of directors of Kookmin Bank, a wholly-owned subsidiary of KB Financial Group, passed a resolution to declare a cash dividend of KRW 570 per common share (total dividend amount: KRW 230,496,096,120), subject to shareholder approval. The record date is December 31, 2014, and in accordance with the Korean Commercial Code, the payment for such dividends (which is subject to change pending shareholder approval) is expected to be made within one month following shareholder approval at the annual general meeting of shareholders of Kookmin Bank, which is expected to be held on March 26, 2015. The entire dividend amount is to be paid to KB Financial Group, which holds all of the common shares of Kookmin Bank.

The contents of the above declaration remain subject to change pending approval at the annual general meeting of shareholders of Kookmin Bank and the results of the audit of Kookmin Bank by its independent auditors.

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.