UNITED STATES

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) June 3, 2014

ALLISON TRANSMISSION HOLDINGS, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code (317) 242-5000

Not Applicable

(Former name or former address, if changed since last report)

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 (see General Instruction A.2. below):

Item 1.01 Entry Into a Material Definitive Agreement.

On June 3, 2014, Allison Transmission Holdings, Inc. (the “Company”), investment funds affiliated with The Carlyle Group and Onex Corporation (the “Selling Stockholders”) and Citigroup Global Markets Inc., as the sole underwriter named in the underwriting agreement (the “Underwriter”), entered into an underwriting agreement (the “Underwriting Agreement”), pursuant to which (i) the Selling Stockholders agreed to sell to the Underwriter, and the Underwriter agreed to purchase from the Selling Stockholders, subject to and upon the terms and conditions set forth therein, 35,000,000 shares of the Company’s common stock and (ii) the Underwriter agreed to sell to the Company, and the Company agreed to repurchase from the Underwriter, subject to and upon the terms and conditions set forth therein, 5,000,000 shares of the Company’s common stock. In addition, pursuant to the Underwriting Agreement, the Selling Stockholders have granted the Underwriter an option, exercisable within 30 days, to purchase up to an additional 5,250,000 shares of common stock on the same terms and conditions.

A copy of the Underwriting Agreement has been attached as Exhibit 1.1 to this Current Report on Form 8-K and is incorporated by reference herein. The foregoing description of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to such Exhibit.

Item 9.01 Financial Statements and Exhibits.

(d) Exhibits.

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 hereunto duly authorized.

EXHIBIT INDEX