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): January 25, 2013

Arrowhead Research Corporation

(Exact name of registrant as specified in its charter)

0-21898

(Commission File Number)

225 South Lake Avenue, Suite 1050, Pasadena, CA 91101

(Address of principal executive offices) (Zip Code)

Registrant’s telephone number, including area code (626) 304-3400

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 8.01. Other Events.

On January 25, 2013, Arrowhead Research Corporation (the “Company”) priced a best efforts public offering for the sale of an aggregate of up to 1.7 million units (each a “Unit”, and collectively, the “Units”) at a price of $2.12 per Unit (the “Offering”). Each Unit consists of (x) one share of common stock, par value $0.001 per share, of the Company (“Common Stock”) and (y) one warrant (each a “Warrant” and collectively, the “Warrants”) to purchase 0.5 shares of Common Stock.

The Warrants will be exercisable immediately on the date of issuance at an initial exercise price of $2.14 per share and expire on the fifth anniversary of the date of issuance. The exercise price of the Warrants may be lowered in the future pursuant to an anti-dilution feature contained in the Warrants. The Warrants will be issued separately from the Common Stock included in the Units and may be transferred separately immediately thereafter. The Warrants will not be listed on any national securities exchange or other trading market, and no trading market for such Warrants is expected to develop.

Dawson James Securities, Inc. (“Dawson James”) is acting as sole book-running manager for the Offering pursuant to a placement agent agreement, dated as of November 28, 2012, between Dawson James and the Company, as amended on January 29, 2013 (the “Placement Agent Agreement”). Additionally, the Company is placing a portion of the Units directly to certain investors pursuant to subscription agreements between the Company and such investors.

The Warrants, the shares of Common Stock issued as part of the Units (the “Common Shares”) and the shares of Common Stock underlying the Warrants (the “Warrant Shares” and, collectively with the Common Shares, the “Shares”) are being issued pursuant to the Company’s effective registration statement on Form S-3 (Registration No. 333-176159) (the “Registration Statement”).

Dawson James will use its best efforts to arrange for the sale of the Units for which it confirmed sales. The sale of the Units is expected to close on or about January 30, 2013. The Company will pay Dawson James an aggregate placement fee equal to (i) 6% of the gross proceeds related to Units placed by Dawson James (excluding sales, if any, made to preexisting investors of the Company) and (ii) 1% of the gross proceeds related to all Units issued in the Offering. The Company will also reimburse Dawson James up to $35,000 for its accountable expenses.

In addition, the Company will issue to Dawson James a warrant (the “Dawson James Warrant”) to purchase approximately 40,000 shares of Common Stock. The Dawson James Warrant will be identical to the Warrants, except that (i) the exercise price will be $2.65, (ii) the expiration date will be August 17, 2016, (iii) the Dawson James Warrant will not be issued pursuant to the Registration Statement and are therefore restricted securities, (iv) the Dawson James Warrant and any shares of Common Stock issued upon any exercise of such warrant will not be transferable by the Dawson James for a period of 180 days from the closing date of the Offering, except as expressly permitted by FINRA Rule 5110(g)(2) and (v) the Dawson James Warrant will not have anti-dilution protection.

The foregoing description of the Offering is a summary only, is not intended to be complete, and is qualified in its entirety by reference to the Placement Agent Agreement, form of subscription agreement, form of warrant and form of Dawson James Warrant, filed as exhibits to this Current Report on Form 8-K.

Ropes & Gray LLP, counsel to the Company, has issued an opinion to the Company, dated as of January 25, 2013, regarding the Shares and Warrants to be sold in the Offering. A copy of the opinion is filed as Exhibit 5.1 to this Current Report on Form 8-K.

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.

Date: January 30, 2013