Exhibit 5.1

May 24, 2013

Ironwood Pharmaceuticals, Inc.

301 Binney Street

Cambridge, Massachusetts  02142

Re: Registration Statement on Form S-3 (Registration No. 333-179430)

Ladies and Gentlemen:

We have acted as counsel to Ironwood Pharmaceuticals, Inc., a Delaware corporation (the “Company”) in connection with its entry into the Underwriting Agreement, dated May 21, 2013, among the Company and the underwriters named therein (the “Underwriting Agreement”) pursuant to which the Company may issue and sell up to 12,075,000 shares of the common stock, $.001 par value (the “Shares”), including the additional 1,575,000 shares the underwriters have the option to purchase under the Underwriting Agreement, of the Company pursuant to the above-referenced registration statement (as amended through the date hereof, the “Registration Statement”), filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933 (the “Securities Act”).

In connection with this opinion letter, we have examined such certificates, documents and records and have made such investigation of fact and such examination of law as we have deemed appropriate in order to enable us to render the opinions set forth herein.  In conducting such investigation, we have relied, without independent verification, upon certificates of officers of the Company, public officials and other appropriate persons.

The opinions expressed below are limited to the Delaware General Corporation Law.

Based upon and subject to the foregoing, we are of the opinion that the Shares have been duly authorized and, when issued and delivered pursuant to the Underwriting Agreement against payment of the consideration set forth therein, will be validly issued, fully paid and non-assessable.

We hereby consent to your filing this opinion as an exhibit to the Registration Statement and to the use of our name therein and in the related prospectus under the caption “Legal Matters.”  In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Sections 7 of the Securities Act or the rules and regulations of the Commission thereunder.