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): December 21, 2012 (December 20, 2012)

AG Mortgage Investment Trust, Inc.

(Exact name of Registrant as specified in its charter)

245 Park Avenue, 26th floor

New York, New York 10167

(212) 692-2000

(Address, including zip code, and telephone number, including area code, of registrant’s

principal executive offices)

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:

On December 20, 2012, AG Mortgage Investment Trust, Inc. (the “Company”) and AG REIT Management, LLC entered into an Underwriting Agreement (the “Underwriting Agreement”) with Credit Suisse Securities (USA) LLC, Merrill Lynch, Pierce, Fenner & Smith Incorporated, Nomura Securities International, Inc. and Stifel, Nicolaus & Company, Incorporated, as the representatives of the several underwriters named on Schedule A to the Underwriting Agreement (the “Underwriters”), relating to (i) the issuance and sale of 3,750,000 shares of the Company’s common stock, par value $0.01 per share, to the Underwriters and (ii) the grant by the Company to the Underwriters of an option to purchase all or part of 562,500 additional shares of the Company’s common stock (together, the “Shares”). The closing of the offering, which is subject to customary closing conditions, is expected to occur on December 26, 2012.

The Shares will be issued pursuant to the Company’s shelf registration statement on Form S-3 (File No. 333-182671), which was declared effective by the Securities and Exchange Commission on July 20, 2012.

The Underwriting Agreement is filed as Exhibit 1.1 to this Current Report on Form 8-K, and this description of the material terms of the Underwriting Agreement is qualified in its entirety by reference to such exhibit. For a more detailed description of the Underwriting Agreement, see the disclosure under the caption “Underwriting” contained in the Company’s Prospectus Supplement, dated December 20, 2012, which has been filed with the Securities and Exchange Commission pursuant to Rule 424(b) under the Securities Act of 1933, as amended, which disclosure is hereby incorporated by reference.

In connection with the filing of the Underwriting Agreement, the Company is filing as Exhibits 5.1 and 8.1 hereto opinions of its counsel, Hunton & Williams LLP.

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SIGNATURE

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.

INDEX TO EXHIBITS