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):

May 21, 2013

IRONWOOD PHARMACEUTICALS, INC.

(Exact Name of Registrant as Specified in Charter)

(617) 621-7722

(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):

o    Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o    Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o    Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o    Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 8.01 Other Events

On May 21, 2013, Ironwood Pharmaceuticals, Inc. (the “Company”) entered into an underwriting agreement (the “Underwriting Agreement”) with the several underwriters set forth in Schedule 1 thereto (collectively, the “Underwriters”), for whom J.P. Morgan and BofA Merrill Lynch are acting as representatives, related to a public offering (the “Offering”) of 10,500,000 shares of the Company’s Class A common stock, par value $0.001 par value per share, at a price to the public of $13.00 per share pursuant to a registration statement on Form S-3 (File No. 333-179430) and a related prospectus supplement.  In addition, the Company granted the Underwriters an option exercisable for 30 days from the date of the Underwriting Agreement to purchase, at the public offering price less any underwriting discounts and commissions, up to an additional 1,575,000 shares of Class A common stock.  The foregoing description of the Underwriting Agreement is qualified in its entirety by reference to the Underwriting Agreement, which is filed as Exhibit 1.1 hereto and incorporated herein by reference.  J.P. Morgan and BofA Merrill Lynch served as joint book-running managers and as representatives of the Underwriters for the Offering, and Morgan Stanley served as book-running manager for the Offering. Cowen and Company, Ladenburg Thalmann & Co. Inc. and Mizuho Securities served as co-managers for the Offering. The Offering closed on May 24, 2013. On May 21, 2013, the Company issued a press release announcing the pricing of the Offering.  A copy of the Company’s press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference.

An opinion dated May 24, 2013 regarding the validity of the shares of Class A common stock offered in the Offering is filed as Exhibit 5.1 to this Current Report on Form 8-K.

Item 9.01 Financial Statements and 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