St. Louis received pitchers Richard Fitts and Brandon Clarke, along with cash considerations and a player to be named later.
The 36-year-old Gray went 14-8 with a 4.28 earned run average through 32 starts last season. He struck out 201 and walked 35 through 180.2 innings of work - making it back-to-back seasons with at least 200 strikeouts for the first time in his career.
Gray's 185 hits given up were the second most in his career (2014 - 187 hits in 219 innings).
However, Gray's 5.29 strikeouts-to-walk ratio was the best in the National League last season and a career-best. It was also just below Red Sox ace and Cy Young candidate Garrett Crochet's 5.54.
Gray, a three-time All-Star, will be entering his 14th season and is in the final year of his three-year, $75 million contract, which has a mutual option for 2027. He will earn $11 million this season with his sixth team.
He'll be joining a Boston starting rotation that ranked 12th in ERA (3.92), 12th in strikeouts (805), 22nd in hits (833), and 25th in walks (301).
Through 339 games (330 starts), Gray has posted a 3.58 ERA with 1,925 strikeouts, giving up 621 walks and 1,675 hits through 1,918 innings, including seven complete games and five complete-game shutouts.
He spent the first four-and-a-half years of his career with the then Oakland Athletics before being traded to the New York Yankees during the 2017 season, where he spent the following season.
Gray would then spend three years with the Cincinnati Reds (2019-2021), two with the Minnesota Twins (2022-2023), and two in St. Louis.