TScan Therapeutics Inc

FOR THE 2025 ANNUAL MEETING OF STOCKHOLDERS

TO BE HELD ON JUNE 30, 2025

This proxy statement (this “Proxy Statement”) contains information about the 2025 Annual Meeting of Stockholders (the “Annual Meeting”) of TScan Therapeutics, Inc. (“TScan,” the “Company,” “we,” “us,” and “our”), which will be held on June 30, 2025, at 8:30 a.m. Eastern Time. The board of directors of the Company (the “board of directors”) is using this Proxy Statement to solicit proxies for use at the Annual Meeting. The mailing address of our principal executive office is TScan Therapeutics, Inc., 830 Winter Street, Waltham, MA 02451.

In order to attend the Annual Meeting virtually, you must log into the meeting atwww.virtualshareholdermeeting.com/TCRX2025. We encourage you to access the Annual Meeting before it begins. Onlinelog-in to access the Annual Meeting will start shortly before the meeting on June 30, 2025. You will be required to complete the registration by entering the control number provided in the Notice of Internet Availability of Proxy Materials (“Notice”) or the proxy card, your name and your email address atwww.virtualshareholdermeeting.com/TCRX2025. Beneficial owners of shares held in street name will need to register as well, and also follow the instructions provided in the voting instructions form by the broker, bank or other nominee that holds their shares. Please see the “General Information” section of this Proxy Statement for more details regarding the logistics of the virtual Annual Meeting. The Annual Meeting will be held virtually only. There will be no physical meeting location, and you will not be able to attend the Annual Meeting in person.

All properly submitted proxies will be voted in accordance with the instructions contained in those proxies. If no instructions are specified, the proxies will be voted in accordance with the recommendation of our board of directors with respect to each of the matters set forth in the accompanying notice of meeting. You may revoke your proxy at any time before it is exercised at the meeting by giving our corporate secretary written notice to that effect.

We made this Proxy Statement and our Annual Report on Form 10-K for the fiscal year ended December 31, 2024 (the “2024 Annual Report”) available to stockholders on or about May 14, 2025.

We are an “emerging growth company” under applicable federal securities laws and therefore permitted to conform with certain reduced public company reporting requirements. As an emerging growth company, we provide in this Proxy Statement the scaled disclosure permitted under the Jumpstart Our Business Startups Act of 2012, including the compensation disclosures required of a “smaller reporting company,” as that term is defined in Rule 12b-2 promulgated under the Securities Exchange Act of 1934, as amended (the “Exchange Act”). In addition, as an emerging growth company, we are not required to conduct votes seeking approval, on an advisory basis, of the compensation of our named executive officers or the frequency with which such votes must be conducted. We would cease to be an emerging growth company on the date that is the earliest of (i) the last day of the fiscal year in which we have total annual gross revenue of $1.235 billion or more; (ii) December 31, 2026; (iii) the date on which we have issued more than $1.0 billion in nonconvertible debt during the previous three years; or (iv) the last day of the fiscal year in which we are deemed to be a large accelerated filer under the rules of the Securities and Exchange Commission, which means the market value of our voting andnon-voting common stock that is held bynon-affiliates exceeds $700 million as of the prior June 30th. Even after we are no longer an “emerging growth company,” we may remain a “smaller reporting company.”

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting of Stockholders to be Held on June 30, 2025

This Proxy Statement and our 2024 Annual Report are available for viewing, printing and downloading atwww.virtualshareholdermeeting.com/TCRX2025.

A copy of our Annual Report on Form 10-K for the fiscal year ended December 31, 2024, as filed with the Securities and Exchange Commission, except for exhibits, will be furnished without charge to any stockholder upon written request to TScan Therapeutics, Inc., 830 Winter Street, Waltham, MA 02451, Attention: Corporate Secretary. This Proxy Statement and our Annual Report on Form 10-K for the fiscal year ended December 31, 2024, are also available on the SEC’s website at www.sec.gov.

PROXY STATEMENT

FOR THE 2025 ANNUAL MEETING OF STOCKHOLDERS