Exhibit 5.1

October 7, 2025

Adicet Bio, Inc.

131 Dartmouth Street, Floor 3

Boston, MA 02116

Re: Securities Registered under Registration Statement on Form S-3

We have acted as counsel to you in connection with your filing of a Registration Statement on Form S-3 (File No. 333-285609) (as amended or supplemented, the “Registration Statement”) filed on March 6, 2025 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 offer by Adicet Bio, Inc., a Delaware corporation (the “Company”), of any combination of securities of the types specified therein. The Registration Statement was declared effective by the Commission on March 14, 2025. Reference is made to our opinion letter dated March 6, 2025 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 October 7, 2025 by the Company with the Commission pursuant to Rule 424 under the Securities Act. The Prospectus Supplement relates to the offering by the Company of up to (i) 70,001,000 shares (the “Primary Shares”) of the Company’s common stock, par value $0.0001 per share (the “Common Stock”) and (ii) pre-funded warrants (the “Pre-Funded Warrants” and, together with the Shares (as defined below), the “Securities”) to purchase up to 10,000,000 shares of Common Stock (such shares issuable upon exercise of the Pre-Funded Warrants, the “Pre-Funded Warrant Shares” and together with the Primary Shares, the “Shares”) covered by the Registration Statement. The Primary Shares and Pre-Funded Warrants are being sold to the several underwriters named in, and pursuant to, an underwriting agreement among the Company and such underwriters (the “Underwriting 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.

The opinions set forth below are limited to the Delaware General Corporation Law, and with respect to opinion paragraph 2, the law of the State of New York.

Based on the foregoing, we are of the opinion that:

The opinions expressed above are subject to bankruptcy, insolvency, fraudulent transfer, reorganization, moratorium and other similar laws of general application affecting the rights and remedies of creditors and to general principles of equity.

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).

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 and Pre-Funded Warrants (the “Current Report”), which is incorporated by reference in the Registration Statement. We hereby consent to the inclusion of this opinion as Exhibit 5.1 to the Current Report and its incorporation by reference in the Registration Statement and to the reference to our firm under the caption “Legal Matters” in the Prospectus Supplement. 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.

Very truly yours,

/s/ Goodwin Procter LLP

GOODWIN PROCTER LLP