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):

September 7, 2012

American Assets Trust, Inc.

(Exact name of registrant as specified in its charter)

(858) 350-2600

Registrant’s telephone number, including area code:

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:

Item 1.01 Entry into a Material Definitive Agreement.

On September 7, 2012, American Assets Trust, Inc. (the “Company”) and American Assets Trust, L.P. (the “Operating Partnership”) entered into the Third Amendment to Credit Agreement (the “Third Amendment”) with the lenders from time to time party thereto, Bank of America, N.A., as Administrative Agent, Swing Line Lender and L/C Issuer, and other entities named therein, that amends the provisions of the Company’s revolving credit facility to, among other things, allow the Company’s consolidated total secured indebtedness to be up to 55% of the Company’s secured total asset value for the period commencing upon the date that a material acquisition (generally, greater than $100,000,000) is consummated through and including the last day of the third full fiscal quarter that follows such date, subject to certain conditions set forth in the Credit Agreement, as amended.

Certain of the banks and financial institutions that are parties to the Third Amendment as well as their respective affiliates have in the past provided, are currently providing, and in the future may continue to provide investment banking, commercial banking and other financial services to the Company and its affiliates in the ordinary course of business for which they have received and will receive customary compensation.

The foregoing descriptions of the Third Amendment does not purport to be complete and is qualified in its entirety by reference to the full text of the Third Amendment, which is filed as Exhibit 10.1 hereto, and is incorporated herein by reference.

Item 2.03 Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of Registrant

The terms of the direct financial obligations are summarized in Item 1.01 of this Form 8-K and are incorporated by reference into this Item 2.03.

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