In a deal that was made official on Thanksgiving Day 2005, Beckett was traded to the Boston Red Sox along with third baseman Mike Lowell and relief pitcher Guillermo Mota for minor league shortstop Hanley Ramírez (the top prospect in the Red Sox system at the time) and pitchers Aníbal Sánchez, Jesús Delgado and Harvey García.
Beckett and Lowell were among a prominent list of veterans the Marlins traded in an effort to shed salary.
This was organizationally termed a market correction.
 In his first start with the Red Sox, Beckett allowed only one run in seven innings to pick up the win against the Texas Rangers on April 5, 2006.

Beckett was the first Red Sox pitcher to hit a home run in 35 years — since the advent of the designated hitter rule — when he took Phillies' pitcher Brett Myers deep during an interleague game on May 20, 2006.
On July 18, he signed a three-year, $30 million contract extension with a $10 million club option for 2010.
Beckett completed his first season with the Red Sox with a record of 16–11 and a 5.01 ERA.
In 204 ​²⁄₃ innings, he gave up 191 hits and struck out 158 batters while walking 74.
He also allowed 36 home runs, tied for second most in the majors.

At the start of the 2007 season, Beckett adjusted to throwing more breaking pitches and fewer fastballs.
At the same time, he learned to locate his pitches rather than simply get strikes by power.
He reduced his walks and home runs allowed by nearly half, contributing to his success in 2007.
Beckett became one of six Red Sox pitchers in history to win their first seven starts.
George Winter and Mickey Harris both won their first seven starts in a season, and Babe Ruth, Dave Ferriss and Roger Moret all won their first eight starts in a season.
After a strong first half, posting a 12–2 record with a 3.44 ERA, he was selected to the American League team in the 2007 MLB All-Star Game.
Beckett earned the victory in the game after pitching two innings, giving up only one hit, and striking out two.
Beckett became the first pitcher to win 20 games in a season since 2005, finishing the season with a record of 20–7, a 3.27 ERA, 194 strikeouts, a 1.14 WHIP, and only 40 walks and 17 home runs allowed.
He finished second in AL Cy Young Award voting, behind CC Sabathia.

Beckett started the first game of the 2007 American League Championship Series (ALCS) and pitched six innings for the win and in game five he struck out 11 in eight innings.
Beckett's 2–0 record and 1.93 ERA against the Cleveland Indians earned him the 2007 ALCS MVP Award.
He then started and won the first game of the 2007 World Series against the Colorado Rockies, pitching seven innings, allowing one run on six hits, and striking out nine batters.
Boston would go on to sweep the Rockies in the series.

On May 8, 2008, Beckett recorded his 1,000th career strikeout, when Brandon Inge of the Detroit Tigers struck out swinging in the seventh inning.
He was 12–10 with a 4.03 ERA in 27 starts.
In game three of the 2008 American League Division Series against the Anaheim Angels he allowed four runs in five innings He then made two starts in the 2008 ALCS against the Tampa Bay Rays and was 1–0 but had a high 9.64 ERA.

In 2009, during his first Opening Day start with the Red Sox, Beckett struck out 10 through seven innings allowing only one run and two hits in a 5–3 victory over the Rays.
On April 14, 2009, MLB fined and suspended him six games for intentionally throwing over the head of Angels outfielder Bobby Abreu.
Beckett later appealed the suspension and got it reduced to five games.
On June 20, 2009, he recorded his first complete game shutout in three years against the Atlanta Braves in a duel against former Red Sox pitcher Derek Lowe.
The shutout was his first in a Red Sox uniform.
On July 5, 2009, he was selected to represent Boston in the 2009 MLB All Star Game.

Beckett recorded his 100th career win on July 12, 2009, pitching a complete game shutout against the Kansas City Royals at Fenway Park.
On August 7, 2009, against the New York Yankees, he engaged in a pitching duel with former Marlins
In a deal that was made official on Thanksgiving Day 2005, Beckett was traded to the Boston Red Sox along with third baseman Mike Lowell and relief pitcher Guillermo Mota for minor league shortstop Hanley Ramírez (the top prospect in the Red Sox system at the time) and pitchers Aníbal Sánchez, Jesús Delgado and Harvey García. Beckett and Lowell were among a prominent list of veterans the Marlins traded in an effort to shed salary. This was organizationally termed a market correction.  In his first start with the Red Sox, Beckett allowed only one run in seven innings to pick up the win against the Texas Rangers on April 5, 2006.
Beckett was the first Red Sox pitcher to hit a home run in 35 years — since the advent of the designated hitter rule — when he took Phillies' pitcher Brett Myers deep during an interleague game on May 20, 2006. On July 18, he signed a three-year, $30 million contract extension with a $10 million club option for 2010. Beckett completed his first season with the Red Sox with a record of 16–11 and a 5.01 ERA. In 204 ​²⁄₃ innings, he gave up 191 hits and struck out 158 batters while walking 74. He also allowed 36 home runs, tied for second most in the majors.
At the start of the 2007 season, Beckett adjusted to throwing more breaking pitches and fewer fastballs. At the same time, he learned to locate his pitches rather than simply get strikes by power. He reduced his walks and home runs allowed by nearly half, contributing to his success in 2007. Beckett became one of six Red Sox pitchers in history to win their first seven starts. George Winter and Mickey Harris both won their first seven starts in a season, and Babe Ruth, Dave Ferriss and Roger Moret all won their first eight starts in a season. After a strong first half, posting a 12–2 record with a 3.44 ERA, he was selected to the American League team in the 2007 MLB All-Star Game. Beckett earned the victory in the game after pitching two innings, giving up only one hit, and striking out two. Beckett became the first pitcher to win 20 games in a season since 2005, finishing the season with a record of 20–7, a 3.27 ERA, 194 strikeouts, a 1.14 WHIP, and only 40 walks and 17 home runs allowed. He finished second in AL Cy Young Award voting, behind CC Sabathia.
Beckett started the first game of the 2007 American League Championship Series (ALCS) and pitched six innings for the win and in game five he struck out 11 in eight innings. Beckett's 2–0 record and 1.93 ERA against the Cleveland Indians earned him the 2007 ALCS MVP Award. He then started and won the first game of the 2007 World Series against the Colorado Rockies, pitching seven innings, allowing one run on six hits, and striking out nine batters. Boston would go on to sweep the Rockies in the series.
On May 8, 2008, Beckett recorded his 1,000th career strikeout, when Brandon Inge of the Detroit Tigers struck out swinging in the seventh inning. He was 12–10 with a 4.03 ERA in 27 starts. In game three of the 2008 American League Division Series against the Anaheim Angels he allowed four runs in five innings He then made two starts in the 2008 ALCS against the Tampa Bay Rays and was 1–0 but had a high 9.64 ERA.
In 2009, during his first Opening Day start with the Red Sox, Beckett struck out 10 through seven innings allowing only one run and two hits in a 5–3 victory over the Rays. On April 14, 2009, MLB fined and suspended him six games for intentionally throwing over the head of Angels outfielder Bobby Abreu. Beckett later appealed the suspension and got it reduced to five games. On June 20, 2009, he recorded his first complete game shutout in three years against the Atlanta Braves in a duel against former Red Sox pitcher Derek Lowe. The shutout was his first in a Red Sox uniform. On July 5, 2009, he was selected to represent Boston in the 2009 MLB All Star Game.
Beckett recorded his 100th career win on July 12, 2009, pitching a complete game shutout against the Kansas City Royals at Fenway Park. On August 7, 2009, against the New York Yankees, he engaged in a pitching duel with former Marlins 
Josh Beckett