Form 8-K TCR2 THERAPEUTICS INC. For: Apr 30

Exhibit 5.1

April 30, 2021

TCR² Therapeutics Inc.

100 Binney Street, Suite 710

Cambridge, MA 02142

We have acted as counsel to you in connection with your filing of an automatic shelf registration statement on Form S-3 (as amended or supplemented, the “Registration Statement”) filed on March 16, 2021 with the Securities and Exchange Commission (the “Commission”) pursuant to the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration of the offering by TCR2 Therapeutics Inc., a Delaware corporation (the “Company”), of an indeterminate amount of any combination of securities of the types specified therein. Reference is made to our opinion letter dated March 16, 2021 and included as Exhibit 5.1 to the Registration Statement. We are delivering this supplemental opinion letter in connection with the prospectus supplement (the “Prospectus Supplement”) filed on April 30, 2021 by the Company with the Commission pursuant to Rule 424(b) under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to $100,000,000 in shares (the “Shares”) of the Company’s common stock, par value $0.0001 per share (“Common Stock”), covered by the Registration Statement. The Shares are being offered and sold by the sales agent named in, and pursuant to, the sale agreement between the Company and such sales agent.

We have reviewed such documents and made such examination of law as we have deemed appropriate to give the opinion set forth below. We have relied, without independent verification, on certificates of public officials and, as to matters of fact material to the opinion set forth below, on certificates of officers of the Company.

For purposes of the opinion set forth below, we have assumed that the Shares are issued for a price per share equal to or greater than the minimum price authorized by the Company’s board of directors or committee thereof prior to the date hereof (the “Minimum Price”) and 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 that may be issued for the Minimum Price.

For purposes of the opinion set forth below, we refer to the following as “Future Approval and Issuance”: (a) the approval by the Company’s board of directors (or a duly authorized committee of the board of directors) of the issuance of the Shares (the “Approval”) and (b) the issuance of the Shares in accordance with the Approval and the receipt by the Company of the consideration (which shall not be less than the par value of such Shares) to be paid in accordance with the Approval.

TCR² Therapeutics Inc.

April 30, 2021

Page 2

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 Future Approval and Issuance, will be validly issued, fully paid and nonassessable.

This opinion is being furnished to you for submission to the Commission as an exhibit to the Company’s Current Report on Form 8-K relating to the Shares (the “Current Report”), which is incorporated by reference in the Registration Statement. We hereby consent to the filing of this opinion letter as an exhibit to the Current Report and its incorporation by reference and the reference to our firm in that report. In giving our consent, we do not admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.