The Braves and left-hander Billy Wagner come to terms on a $7 million, one-year deal for the southpaw to become the club’s closer, replacing Rafael Soriano and Mike Gonzalez on December 2, 2009. The Boston Red Sox claimed Wagner off waivers from the Mets. After initial reports suggested Wagner would invoke his no-trade clause to veto a trade, he agreed to be traded on August 25 for Chris Carter and Eddie Lora, with the added stipulation that the Red Sox could not exercise his $8 million option for 2010, but could offer him salary arbitration. The Red Sox did offer Wagner arbitration, but he declined so the Red Sox received the first-round draft pick from the team that signed Wagner (Atlanta Braves) and a sandwich pick in the 2010 rookie draft.
The six-time All-Star, who missed most of the 2009 season due to elbow surgery, was traded by the Mets to the Red Sox in late August after the reliever showed he still has a live fastball in his initial appearance off the disabled list in New York.
On April 30, 2010, Wagner revealed that he would retire at the end of the 2010 season to spend more time with his family. In a game against the Detroit Tigers on June 25, Wagner achieved his 400th career save. After the game, he told reporters that he still planned to retire after the 2010 season. On July 11, Wagner was selected as an injury replacement to the 2010 National League All Star roster, which he declined due to an ankle injury.
He played his final regular season game on October 3, 2010, and struck out the final four batters he faced – the last three of whom struck out looking. He concluded his final major league regular season with a career-best 1.43 ERA. Wagner made his final major league appearance on October 8 in Game 2 of the NLDS against the San Francisco Giants. Wagner suffered an injury to his left oblique and left the game after facing just two batters. The Braves eventually lost the series before Wagner could recover.