TScan Therapeutics, Inc.

FOR THE SPECIAL MEETING OF STOCKHOLDERS

TO BE HELD ON NOVEMBER 20, 2026

This proxy statement (this “Proxy Statement”) contains information about the Special Meeting of Stockholders (the “Special Meeting”) of TScan Therapeutics, Inc. (“TScan,” the “Company,” “we,” “us,” and “our”), which will be held on November 20, 2026, 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 Special Meeting. The mailing address of our principal executive office is TScan Therapeutics, Inc., 830 Winter Street, Waltham, MA 02451.

In order to attend the Special Meeting virtually, you must log into the meeting atwww.virtualshareholdermeeting.com/TCRX2026SM. We encourage you to access the Special Meeting before it begins. Onlinelog-in to access the Special Meeting will start shortly before the meeting on November 20, 2026. You will be required to complete the registration by entering the control number provided in the Proxy Materials, your name and your email address atwww.virtualshareholdermeeting.com/TCRX2026SM. 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 Special Meeting. The Special Meeting will be held virtually only. There will be no physical meeting location, and you will not be able to attend the Special 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 available to stockholders on or about October 26, 2026.

Important Notice Regarding the Availability of Proxy Materials for the Special Meeting of Stockholders to be Held on November 20, 2026

This Proxy Statement is available for viewing, printing and downloading atwww.virtualshareholdermeeting.com/TCRX2026SM.

A copy of our Annual Report, 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 are also available on the SEC’s website at www.sec.gov.

PROXY STATEMENT

FOR THE SPECIAL MEETING OF STOCKHOLDERS