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

August 10, 2011

Date of Report (Date of earliest event reported)

AGENUS INC.

(Exact name of registrant as specified in its charter)

781-674-4400

(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 (see General Instruction A.2. below):

On August 10, 2011, Agenus Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with William Blair & Company, L.L.C., acting as representative of the underwriters named therein (collectively, the “Underwriters”) pursuant to which the Company agreed to offer and sell 13,725,491 shares of its common stock in an underwritten offering at a price of $0.51 per share (the “Offering”). The Company expects that its chief executive officer will purchase shares in the Offering at the offering price and on the same terms as the other investors purchasing shares in the Offering. The Company expects to receive approximately $6.3 million in net proceeds from the Offering, after underwriting fees and discounts and other offering expenses. The shares are expected to be delivered to the Underwriters on or about August 16, 2011, subject to the satisfaction of customary closing conditions.

The shares are being offered and sold in the Offering pursuant to the Company’s currently effective shelf registration statement on Form S-3 (File No. 333-164481) (the “Registration Statement”), as supplemented by a prospectus supplement dated August 10, 2011.

The above description of the Underwriting Agreement is qualified in its entirety by the full text of the Underwriting Agreement, which is attached to this report as Exhibit 1.1 and is incorporated by reference herein.

A copy of the opinion of Choate, Hall and Stewart LLP relating to the legality of the issuance and sale of the shares pursuant to the Offering is attached hereto as Exhibit 5.1.

On August 11, 2011, the Company issued a press release announcing the pricing of the Offering. A copy of the press release is attached hereto as Exhibit 99.2.

(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