The 13-year veteran spent his first 12 seasons with the Los Angeles Dodgers. Jansen's 573 games finished are the most among active players. His 391 career saves are second-most among active players and the eighth-most in MLB history.
The right-handed relief pitcher has a career 2.46 ERA in 766 appearances.
Jansen is a three-time All-Star and two-time Reliever of the Year Award winner. He also won the World Series in 2020 with the Dodgers.
The Red Sox owned the fifth-worst bullpen in baseball, with a 4.59 ERA last season. They signed right-handed relief pitcher Chris Martin last week. Martin, 36, went 4-1 with a 3.05 ERA in 60 games last season, which he split between the Dodgers and Chicago Cubs.