| Date | Roster move | Ref. |
| October 3 | The team recalled Chris Murphy from Worcester, and activated Roman Anthony, Brennan Bernardino, Isaiah Campbell, Kristian Campbell, Cooper Criswell, José De León, Richard Fitts, Jhostynxon García, Lucas Giolito, Jordan Hicks, Dustin May, Jovani Morán, and Luis Perales from Worcester, the 10-day injured list, or the 15-day injured list. | |
| October 10 | The team signed free agents Sebastian Rondon, Steven Garcia, Hicter Ruiz, and Cristopher Silvestre to minor league contracts. | |
| October 13 | The team dismissed Ben Rosenthal as assistant hitting coach. | |
| October 16 | The team sent Isaiah Campbell and José De León outright to Worcester. | |
| October 23 | The team hired John Soteropulos as assistant hitting coach. | |
| October 27 | The team signed free agent Jason Delay to a minor league contract. | |
| November 2 | Pitchers Steven Matz, Dustin May, and Justin Wilson, and outfielder Rob Refsnyder, each elected free agency. | |
| November 3 | Third baseman Alex Bregman opted out of his contract and pitcher Lucas Giolito's mutual option was declined, resulting in both players becoming free agents. | |
| November 4 | The team re-signed outfielder Jarren Duran to a one-year, $7.7m contract for 2026 after declining the option on his prior contract. Shortstop Trevor Story opted in to the remaining two years of his contract to remain with the team, and the team declined their mutual option for pitcher Liam Hendriks, making him a free agent. | |
| November 6 | The team activated Triston Casas, Kutter Crawford, Hunter Dobbins, Vaughn Grissom, Tanner Houck, Marcelo Mayer, Patrick Sandoval, and Josh Winckowski from the 60-day injured list, and to clear room on the 40-man roster, the team designated Luis Guerrero for assignment. The team re-signed pitcher Cooper Criswell to a one-year major league contract for 2026. | |
| November 18 | The team designated Josh Winckowski and Nathaniel Lowe for assignment, selected Shane Drohan, David Sandlin, and Tyler Uberstine to the Red Sox roster from Worcester, and completed a series of trades: Luis Guerrero was traded to the Tampa Bay Rays for infielder Tristan Gray, Brennan Bernardino was traded to the Colorado Rockies for minor league outfielder Braiden Ward, Chris Murphy was traded to the Chicago White Sox for minor league catcher Ronny Hernandez, and minor league pitcher Alex Hoppe was traded to the Seattle Mariners for minor league catcher Luke Heyman. | |
| November 20 | The team re-signed catcher Connor Wong to a one-year, $1.3m contract for 2026 to avoid arbitration. | |
| November 21 | The team tendered contracts to all 26 remaining unsigned players on the major league roster, excluding Nathaniel Lowe and Josh Winckowski, who became free agents. | |
| November 25 | The team traded pitchers Brandon Clarke and Richard Fitts to the St. Louis Cardinals in exchange for Sonny Gray and cash. | |
| December 4 | The team designated pitcher Cooper Criswell for assignment, and traded minor league prospects Jhostynxon García and Jesus Travieso to the Pittsburgh Pirates in exchange for Johan Oviedo, Tyler Samaniego and Adonys Guzman. | |
| December 9 | The team traded Vaughn Grissom to the Los Angeles Angels in exchange for outfield prospect Isaiah Jackson. | |
| December 10 | The team traded minor league prospect Justin Riemer to the Athletics for minor league pitcher Ryan Watson. Watson was originally a Rule V selection from the Giants. | |
| December 15 | The team traded minor league pitcher Luis Perales to the Washington Nationals for minor league pitcher Jake Bennett. | |
| December 21 | The team traded pitcher Hunter Dobbins and minor league pitching prospects Yhoiker Fajardo and Blake Aita to the St. Louis Cardinals for first baseman Willson Contreras. | |
| December 30 | Infielders Marcelo Mayer and Nate Eaton changed their player numbers to 11 and 18, respectively. | |
| Date | Roster move | Ref. |
| January 6 | The team signed free agent pitcher T.J. Sikkema to a minor league contract and invited him to spring training, and further announced Osvaldo Berrios, Vinny Capra, Jason Delay, Alec Gamboa, Hobie Harris, and Devin Sweet as non-roster invitees to spring training. Boston also avoided arbitration with Kutter Crawford, signing him to a one year contract for $2.75 million. | |
| January 9 | The team reached one-year agreements with their four remaining arbitration-eligible players, avoiding arbitration. | |
| January 21 | The team traded infielder Tristan Gray to the Minnesota Twins for minor league catcher Nate Baez. | |
| January 23 | The team announced Nate Baez, Allan Castro, Max Ferguson, Tayron Guerrero, Nathan Hickey, Seth Martinez, Tyler McDonough, Wyatt Olds, Ronald Rosario, Mikey Romero, Noah Song, Braiden Ward, Jacob Webb, and Jeremy Wu-Yelland as non-roster invitees to Spring Training. | |
| January 31 | The team signed free agent catcher Matt Thaiss to a minor league contract with an invite to spring training. Thaiss previously made 60 major league appearances with the Chicago White Sox and Tampa Bay Rays in 2025. | |
| February 1 | The team traded pitcher Jordan Hicks and pitching prospect David Sandlin to the Chicago White Sox, as well as two players to be named later, in exchange for minor league pitcher Gage Ziehl and a player to be named later. The trade included the Red Sox providing Chicago with $8 million in cash, and in turn the White Sox agreed to cover the remaining $16 million of Hicks' $24 million contract. | |