Exhibit 5.1

September 28, 2012

Insmed Incorporated

9 Deer Park Drive, Suite C,

Monmouth Junction, New Jersey 08852

Ladies and Gentlemen:

This opinion letter is furnished to you in connection with the above-referenced registration statement (the “Registration Statement”), the base prospectus included therein, and the prospectus supplement dated September 28, 2012 (the “Prospectus Supplement”). The Prospectus Supplement relates to the offering by Insmed Incorporated, a Virginia corporation (the “Company”), of 6,304,102 shares of the Company’s common stock, par value $0.01 per share (the “Shares”), which Shares are covered by the Registration Statement. We understand that the Shares are to be offered and sold in the manner described in the Prospectus Supplement.

We have acted as your counsel in connection with the preparation of the Prospectus Supplement. We are familiar with the proceedings taken by the Board of Directors of the Company (the “Board of Directors”) and the Pricing Committee duly appointed by the Board of Directors in connection with the authorization, issuance and sale of the Shares. We have examined all such documents as we considered necessary to enable us to render this opinion, including but not limited to the Registration Statement, the base prospectus included in the Registration Statement, the Prospectus Supplement, the Company’s Articles of Incorporation, as amended, and the Company’s Amended and Restated Bylaws, as amended, each as in effect on the date hereof, certain resolutions of the Board of Directors and the Pricing Committee of the Board of Directors, corporate records, and instruments, and such laws and regulations as we have deemed necessary for purposes of rendering the opinions set forth herein. In our examination, we have assumed the legal capacity of all natural persons, the genuineness of all signatures, the authenticity of all documents submitted to us as certified or photostatic copies, the authenticity of the originals of such latter documents and that the Shares will be issued against payment of valid consideration under applicable law. As to any facts material to the opinions expressed herein, which were not independently established or verified, we have relied upon statements and representations of officers and other representatives of the Company and others.

The opinions expressed below are limited to the Virginia Stock Corporation Act, as amended.

Based upon the foregoing, we are of the opinion that the Shares have been duly authorized and, when issued and delivered by the Company against payment therefor as set forth in the Prospectus Supplement, will be validly issued, fully paid and non-assessable.

We hereby consent to the filing of this opinion as a part of the Registration Statement and to the reference of our firm under the caption “Legal Matters” in the Prospectus Supplement. In giving such consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act of 1933, as amended, or the rules and regulations of the Securities and Exchange Commission. Except as otherwise set forth herein, this opinion may not be used, circulated, quoted or otherwise referred to for any purpose or relied upon by any other person without the express written permission of this firm.