X0306 4 2021-08-06 0 0001829802 Sensei Biotherapeutics, Inc. SNSE 0001594129 Pierce Robert Hamilton C/O SENSEI BIOTHERAPEUTICS, INC. 1405 RESEARCH BLVD, SUITE 125 ROCKVILLE MD 20850 0 1 0 0 Chief Scientific Officer Common Stock 2021-08-06 4 S 0 6460 7.1127 D 89362 I By spouse Common Stock 2021-08-09 4 S 0 6982 7.0603 D 82380 I By spouse Common Stock 2021-08-10 4 S 0 66350 7.1289 D 16030 I By spouse The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $7.04 to $7.20, inclusive. The reporting person undertakes to provide to Sensei Biotherapeutics, Inc., any security holder of Sensei Biotherapeutics, Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (1), (3) and (4) to this Form 4. The reporting person disclaims beneficial ownership of these securities, and this report shall not be deemed an admission that the reporting person is the beneficial owner of the securities for purposes of Section 16 or for any other purpose. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $7.00 to $7.10, inclusive. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $7.00 to $7.42, inclusive. /s/ Mark Ballantyne, Attorney-in-Fact 2021-08-10