Bregman, who has spent the first eight years of his career with the Houston Astros, moves to Boston to give them a right-handed bat in a left-hand-dominated lineup.
The two-time all-star won two world titles with the Astros, and was also involved in the infamous sign-stealing scandal that saw them use methods to inform hitters of the pitch that was coming in.
With the bat, Bregman has a career average of .272, with 26 home runs and 75 RBIs in 2024.
Last season, he also won his first gold glove award at third base, showing how important his defence will be for the Red Sox.
Other suitors were interested in the New Mexico native over the winter, with Detroit reportedly offering him a six-year, $171.5-million deal, whilst the Chicago Cubs were also involved in talks.
Instead, Bregman has chosen the higher average salary at Fenway Park, and his deal reportedly has an opt-out clause after the 2025 and 2026 seasons, leaving him with options if things don't pan out well in Boston.
The Red Sox finished the 2024 season with a 81-81 overall record, enough for third in the AL East and missing out on the post season by five wins.
With Houston, Bregman reached the play-offs in 2024, where they were beaten by Detroit in the AL Wildcard Series.