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):

March 1, 2011

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:

On March 1, 2011, American Assets Trust, Inc. (the “Company”), through the Company’s operating partnership subsidiary, American Assets Trust, L.P., entered into a purchase agreement with an unrelated third party, Two Main Development LLC, to acquire a 364,735 square foot, 16-story, LEED Platinum certified office building located at 100 SW Main Street, in Portland, Oregon (“First & Main”).

The purchase price for First & Main, excluding closing costs, is approximately $129 million, which the Company will fund from proceeds of the Company’s initial public offering. Based upon information provided by the seller, the Company believes that, upon closing, approximately 96% of the total rentable square feet will be leased. Major tenants include the U.S. Veterans Benefit Administration, the Internal Revenue Service, Portland Energy Conservation, Inc. and the U.S. Drug Enforcement Administration.

The acquisition is subject to the Company’s due diligence and other customary closing conditions. The Company can offer no assurances that this acquisition will close on the terms described herein, or at all. Additionally, the transaction is being structured to accommodate a possible tax deferred exchange pursuant to the provisions of Section 1031 of the Internal Revenue Code of 1986 and applicable state revenue and taxation code sections.

The foregoing description of the property acquisition does not purport to be complete and is qualified in its entirety by reference to the full text of the purchase agreement, which is filed as Exhibit 10.1 hereto and is incorporated herein by reference.

On March 3, 2011, the Company issued a press release announcing the agreement to purchase First & Main. A copy of the press release is attached hereto as Exhibit 99.1. The information contained in Item 7.01 of this Current Report, including Exhibit 99.1, is being furnished and shall not be deemed “filed” for any purposes, including for the purposes of Section 18 of the Securities Exchange Act of 1934, as amended, or otherwise subject to the liabilities of that section. Such information shall not be incorporated by reference into any filing of the Company, whether made before or after the date hereof, regardless of any general incorporation language in such filing.

(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.

March 3, 2011

EXHIBIT INDEX