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): February 2, 2011

SEATTLE GENETICS, INC.

(Exact name of registrant as specified in its charter)

Delaware

(State or other jurisdiction of incorporation)

21823 30th Drive SE

Bothell, Washington 98021

(Address of principal executive offices, including Zip Code)

Registrant’s telephone number, including area code: (425) 527-4000

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:

In this report, “Seattle Genetics,” “we,” “us” and “our” refer to Seattle Genetics, Inc.

On February 2, 2011, Seattle Genetics entered into an underwriting agreement (the “Underwriting Agreement”) with Jefferies & Company, Inc. and J.P. Morgan Securities LLC, as representatives of the several underwriters named therein (collectively, the “Underwriters”), relating to the issuance and sale of 10,000,000 shares of our common stock, par value $0.001 per share. The price to the public in this offering is $15.50 per share, and the Underwriters have agreed to purchase the shares from us pursuant to the Underwriting Agreement at a price of $14.6475 per share. The net proceeds to Seattle Genetics from this offering are expected to be approximately $146.1 million, after deducting underwriting discounts and commissions and other estimated offering expenses payable by us. The offering is expected to close on or about February 7, 2011, subject to customary closing conditions. In addition, under the terms of the Underwriting Agreement, we have granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 1,500,000 shares of common stock to cover over-allotments, if any.

The offering is being made pursuant to Seattle Genetics’ effective registration statement on Form S-3 (Registration Statement No. 333-159457) previously filed with the Securities and Exchange Commission and a prospectus supplement thereunder. The Underwriting Agreement is filed as Exhibit 1.1 to this report, and the description of the terms of the Underwriting Agreement is qualified in its entirety by reference to such exhibit. A copy of the opinion of Cooley LLP relating to the legality of the issuance and sale of the shares in the offering is attached as Exhibit 5.1 hereto.

The Underwriting Agreement contains customary representations, warranties and agreements by us, customary conditions to closing, indemnification obligations of Seattle Genetics and the Underwriters, including for liabilities under the Securities Act of 1933, as amended, other obligations of the parties and termination provisions. The representations, warranties and covenants contained in the Underwriting Agreement were made only for purposes of such agreement and as of specific dates, were solely for the benefit of the parties to such agreement, and may be subject to limitations agreed upon by the contracting parties.

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.

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