Outfielder Jackie Bradley Jr. also agreed to an $11 million deal, ensuring Boston will avoid salary arbitration with two of its biggest stars.
Outfielder Andrew Benintendi and left-handed starter Eduardo Rodríguez did not agree to deals and swapped numbers for arbitration. Benintendi asked for $4.15 million and was offered $3.4 million — a big raise from his $717,500 salary in 2019. Rodríguez is seeking $8,975,000 and was offered $8.3 million, a bump from $4,325,000.
Boston also agreed to salaries for relievers Brandon Workman ($3.5 million), Matt Barnes ($3.1 million) and Heath Hembree ($1,612,500)
Betts' deal topped the $26 million agreement last winter for third baseman Nolan Arenado, which led to negotiations for a $260 million, eight-year deal.
A four-time All-Star and four-time Gold Glove winner, Betts is eligible for free agency after this season.
He was voted the American League MVP in 2018, when he hit a major league-leading .346 with 32 homers and 80 RBIs, then helped the Red Sox to the World Series title. He hit .295 last season with 29 homers and 80 RBIs.
Bradley is known for his spectacular plays in the outfield, but his bat has been a liability at times. He is streaky, hitting .315 with five homers and 14 RBIs in June but just .225 for the season as a whole, with 21 homers and 62 RBIs. He led the league in putouts and assists as a center fielder.
Last year was a setback for the 25-year-old Benintendi in virtually every offensive category, raising concerns that he has hit his ceiling. Benintendi batted .266 in 2019 with 13 homers, 68 RBIs and 10 stolen bases.
After years of waiting for Rodríguez to develop into a stalwart of the rotation, he finally emerged as a front-line, if unspectacular, starter. He made every start, topped 200 innings for the first time and had a chance at 20 wins going into the regular season finale. In all, that added up to a 19-6 record, with a 3.81 ERA in 203 1/3 innings, all the best in his career.
Thrust into the de facto closer role by the departure of Craig Kimbrel, Workman emerged as a dependable late-inning reliever. At the age of 30, he posted a 10-1 record with 16 saves and a 1.88 ERA, striking out 104 batters — all the best in his career.
Barnes, 29, has also emerged as a reliable arm in the bullpen, good for 60-plus appearances a year and a lot of strikeouts. Although the right-hander's strikeouts have increased each year, he walked a career-high 38 batters in 64 innings in 2019, a small step back.
Hembree, a 30-year-old right-hander missed time in the middle of the season but had a 3.86 ERA in 45 appearances.
Boston also acquired right-hander Austin Brice from the Miami Marlins in exchange for minor league shortstop Angeudis Santos. Infielder Marco Hernández was designated for assignment.
Brice, a 27-year-old relief pitcher, had a 3.43 ERA in 36 games last season.