SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d) OF THE

SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): December 20, 2013

AMBAC FINANCIAL GROUP, INC.

(Exact name of Registrant as specified in its charter)

One State Street Plaza, New York, New York 10004

(Address of principal executive offices) (Zip Code)

(212) 658-7470

(Registrant’s telephone number, including area code)

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

Ambac Financial Group Inc. (the “Company”) granted restricted stock units (“RSUs”) to the following executive officers (each a “Grantee”) on December 20, 2013 (the “Grant Date”) pursuant to the terms of the Company’s Incentive Compensation Plan. Diana Adams, Chief Executive Officer and President, received 33,932 RSUs; David Trick, Senior Managing Director, Chief Financial Officer and Treasurer, received 24,237 RSUs; and Robert Eisman, Senior Managing Director and Chief Accounting Officer received 9,695 RSUs. Each RSU represents a contingent right to receive one share of the Company’s common stock. The RSUs shall vest in two installments, 50% on the Grant Date and 50% on the first anniversary of the Grant Date (each a “Vesting Date”). RSUs that have become vested shall settle and convert into shares of the Company’s common stock on the earlier of (a) the Grantee’s termination of employment with the Company (other than with respect to a termination for “cause”) or (b) the second anniversary of the applicable Vesting Date. Notwithstanding the foregoing, a number of vested RSUs sufficient to satisfy any tax withholding obligations imposed upon the Company may be converted into shares of common stock and withheld by the Company to satisfy such tax withholding obligations. If a Grantee’s employment with the Company is terminated for any reason prior to the applicable Vesting Date (the “Termination Date”), all RSUs which are not vested shall immediately expire and shall be forfeited and the Grantee shall have no further rights with respect to such RSUs. In the event of a termination of the Grantee for “cause” all RSUs which are vested on the Grantee’s Termination Date shall immediately be forfeited and the Grantee shall have no further rights with respect thereto.

Attached as an exhibit to this filing is a Form of Restricted Stock Unit Agreement pursuant to which each of the Grantee’s received their awards. This summary of the terms of the RSU grants is qualified in its entirety by reference to the full text of the Form of Restricted Stock Unit Agreement attached hereto as Exhibit 10.1. As used herein the term “cause” shall have the meaning set forth in the Form of Restricted Stock Unit Agreement.

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.