EX-5.1

Exhibit 5.1

Goodwin Procter LLP

100 Northern Avenue

Boston, MA 02210

goodwinlaw.com

+1 617 570 1000

July 17, 2023

Magenta Therapeutics, Inc.

300 Technology Square, 8^th Floor

Cambridge, MA 02139

Re:	Securities Being Registered under Registration Statement on Form S-4

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-4 (File No. 333-271917) (as amended or supplemented, the “Registration Statement”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of the offering by Magenta Therapeutics, Inc., a Delaware corporation (the “Company”) of up to 170,843,271 shares (the “Shares”) of the Company’s common stock, $0.001 par value per share. The Shares are being issued to the stockholders of Dianthus Therapeutics, Inc., a Delaware corporation (“Dianthus”) pursuant to the merger contemplated by the Agreement and Plan of Merger, dated as of May 2, 2023, by and among the Company, Dio Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of the Company, and Dianthus (as may be amended and/or restated from time to time, the “Merger Agreement”).

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinions set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinions set forth below, on certificates of officers of the Company. For purposes of the opinion set forth below, we have assumed that no event occurs that causes the number of authorized shares of Common Stock available for issuance by the Company to be less than the number of then unissued Shares.

The opinion set forth below is limited to the Delaware General Corporation Law.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, upon issuance and delivery in exchange for the outstanding shares of capital stock of Dianthus in accordance with the terms of the Merger Agreement, the Shares will be validly issued, fully paid and non-assessable.

This opinion letter and the opinion it contains shall be interpreted in accordance with the Core Opinion Principles as published in 74 Business Lawyer 815 (Summer 2019).