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): August 2, 2012

IDENIX PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

(617) 995-9800

(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:

On August 2, 2012, Idenix Pharmaceuticals, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with J.P. Morgan Securities LLC acting as representative of the several underwriters named therein (collectively, the “Underwriters”), relating to an underwritten public offering of 22,000,000 shares (the “Shares”) of the Company’s common stock, par value $0.001 per share (the “Common Stock”). All of the shares are being sold by the Company. The offering price to the public is $8.00 per share, and the Underwriters have agreed to purchase the Shares from the Company pursuant to the Underwriting Agreement at a price of $7.54 per share. After underwriting discounts and commissions and estimated offering expenses, the Company expects to receive net proceeds of approximately $165.7 million. Under the terms of the Underwriting Agreement, the Company has granted the Underwriters an option, exercisable for 30 days, to purchase up to an additional 3,300,000 shares of Common Stock to cover over-allotments, if any, at the public offering price less the underwriting discounts and commissions.

The Shares will be issued pursuant to an automatic shelf registration statement the Company filed with the U.S. Securities and Exchange Commission (the “SEC”), (File No. 333-182953). A prospectus supplement relating to the offering has been filed with the SEC. The closing of the offering is expected to take place on August 7, 2012, subject to the satisfaction of customary closing conditions.

A copy of the Underwriting Agreement is attached as Exhibit 1.1 hereto and is incorporated herein by reference. The foregoing description of the material terms of the Underwriting Agreement does not purport to be complete and is qualified in its entirety by reference to such exhibit.

A copy of the legal opinion and consent of Wilmer Cutler Pickering Hale and Dorr LLP relating to the Shares is attached as Exhibit 5.1 hereto.

The Company issued a press release on August 2, 2012 announcing the pricing of the public offering. This press release is attached as Exhibit 99.1 hereto and is incorporated herein by reference.

(d) The following exhibits are included in this report:

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SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the Registrant has duly caused this report to be signed by the undersigned hereunto duly authorized.

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