EXHIBIT 5.1

Hogan Lovells US LLP 1735 Market Street, Floor 23 Philadelphia, PA 19103 T +1 267 675 4600 F +1 267 675 4601 www.hoganlovells.com

November 21, 2022

Board of Directors

Sesen Bio, Inc.

245 First Street, Suite 1800

Cambridge, MA 02142

Ladies and Gentlemen:

We are acting as counsel to Sesen Bio, Inc., a Delaware corporation (the “Company”), in connection with its registration statement on Form S-4 (File No. 333-267891) (as may be amended from time to time, the “Registration Statement”), initially filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”), on October 14, 2022, relating to the proposed offering of up to 440,794,480 shares (as such number may be adjusted after giving effect to the reverse stock split contemplated by the Registration Statement) of common stock, par value $0.001 per share, of the Company (the “Shares”), in connection with the merger contemplated by the Agreement and Plan of Merger and Reorganization, dated as of September 20, 2022, by and among the Company, CARISMA Therapeutics Inc., a Delaware corporation, and Seahawk Merger Sub, Inc., a Delaware corporation and a wholly-owned subsidiary of the Company (the “Merger Agreement”).  This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed.  In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including pdfs). As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on. This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Delaware General Corporation Law, as amended.  We express no opinion herein as to any other statutes, rules or regulations.

Based upon, subject to and limited by the foregoing, we are of the opinion that following (i) the effectiveness of the Registration Statement, (ii) the approval by the stockholders of the Company of the issuance of the Shares, (iii) the approval by the stockholders of the Company of the Certificate of Amendment to the Company’s Restated Certificate of Incorporation, as amended, in the form attached as Annex G to the Registration Statement (the “Certificate of Amendment”), and the filing of the Certificate of Amendment with the Secretary of State of the State of Delaware, and (iv) the issuance

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Sesen Bio, Inc. - 2 - November 21, 2022

of the Shares by the Company in accordance with the terms of the Merger Agreement, the Shares will be validly issued, fully paid, and nonassessable.

This opinion letter has been prepared for use in connection with the Registration Statement.  We assume no obligation to advise of any changes in the foregoing subsequent to the effective date of the Registration Statement.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the reference to this firm under the caption “Legal Matters” in the prospectus constituting a part of the Registration Statement.  In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Act.

Very truly yours,

/s/ HOGAN LOVELLS US LLP

HOGAN LOVELLS US LLP