Exhibit 5.1

May 20, 2015

Dicerna Pharmaceuticals, Inc.

87 Cambridgepark Drive

Cambridge, MA 02140

Ladies and Gentlemen:

We refer to the Registration Statement on Form S-3, File No. 333-202687 (the “Registration Statement”), filed by Dicerna Pharmaceuticals, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”), which Registration Statement was declared effective on April 2, 2015. Pursuant to the Registration Statement, the Company is issuing up to 3,162,500 shares (the “Shares”) of its Common Stock, $0.0001 par value per share. The Shares are to be sold by the Company pursuant to an underwriting agreement dated May 20, 2015 (the “Underwriting Agreement”) among the Company and Underwriters named therein.

This opinion letter is being delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

In rendering the opinion set forth in this letter, we have examined and relied upon copies of the Registration Statement, the Company’s base prospectus dated April 2, 2015 (the “Base Prospectus”), the Company’s prospectus supplement dated May 20, 2015 supplementing the Base Prospectus and relating to the Shares (the “Prospectus Supplement”), the Underwriting Agreement, the Company’s certificate of incorporation and the resolutions adopted by the board of directors of the Company and the pricing committee thereof established by such board relating to the Registration Statement and the issuance of the Shares by the Company. We have also examined originals, or copies of originals certified to our satisfaction, of such agreements, documents, certificates and statements of the Company and other corporate documents and instruments, and have examined such questions of law, as we have considered relevant and necessary as a basis for this opinion letter. We have assumed the authenticity of all documents submitted to us as originals, the genuineness of all signatures, the legal capacity of all persons and the conformity with the original documents of any copies thereof submitted to us for examination. As to facts relevant to the opinions expressed herein, we have relied without independent investigation or verification upon, and assumed the accuracy and completeness of, certificates, letters and oral and written statements and representations of public officials and officers and other representatives of the Company.

Sidley Austin (CA) LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in

affiliation with other Sidley Austin partnerships.

Dicerna Pharmaceuticals, Inc.

May 20, 2015

Page 2

Based on and subject to the foregoing and the other limitations, qualifications and assumptions set forth herein, we are of the opinion that:

1. The issuance and sale of 2,750,000 of the Shares covered by the Registration Statement pursuant to the Underwriting Agreement have been duly authorized by the Company, and such Shares will be validly issued, fully paid and non-assessable when certificates representing such Shares shall have been duly executed, countersigned and registered and duly delivered to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof or, if any such Shares are to be issued in uncertificated form, the Company’s books shall reflect the issuance of such Shares to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof, in accordance with the Underwriting Agreement.

2. The issuance and sale of 412,500 of the Shares covered by the Registration Statement in connection with any exercise of the over-allotment option granted by the Company to the underwriters pursuant to the Underwriting Agreement have been duly authorized by the Company, and such Shares will be validly issued, fully paid and non-assessable when certificates representing such Shares shall have been duly executed, countersigned and registered and duly delivered to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof or, if any such Shares are to be issued in uncertificated form, the Company’s books shall reflect the issuance of such Shares to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof, in accordance with the Underwriting Agreement.

For the purposes of paragraph 2 of this opinion letter, we have assumed that, at the time of the issuance, sale and delivery of Shares pursuant to the over-allotment option: (i) the authorization thereof by the Company will not have been modified or rescinded, and there will not have occurred any change in law affecting the validity thereof; and (ii) the certificate of incorporation and bylaws of the Company, as currently in effect, will not have been modified or amended and will be in full force and effect.

This opinion letter is limited to the General Corporation Law of the State of Delaware. We express no opinion as to the laws, rules or regulations of any other jurisdiction, including, without limitation, the federal laws of the United States of America or any state securities or blue sky laws.

We hereby consent to the filing of this opinion letter as an Exhibit to the Company’s Current Report on Form 8-K to be filed with the Commission on or after the date hereof, which Form 8-K will be incorporated by reference into the Registration Statement, and to all references to our Firm under the caption “Legal Matters” in the Prospectus Supplement. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act.