In the 2018 World Series against the Los Angeles Dodgers, Pearce hit a game-tying solo home run off of Dodgers closer Kenley Jansen in the top of the eighth inning of Game 4. In the next inning, Pearce came up with the bases loaded and hit a base–clearing double, putting the Red Sox up 8–4 in a game they would go on to win 9–6. In Game 5, Pearce hit a two-run home run in the first inning off of former Cy Young Award winner Clayton Kershaw, and added a second home run in the eighth inning off Pedro Báez to extend the lead to 5–1. The Red Sox would go on to win the game and the series, giving Pearce his first World Series ring. He became the first Red Sox player to hit three home runs in the World Series since Carl Yastrzemski in 1967, and in three fewer games. He joined Babe Ruth and Ted Kluszewski as the only players 35 or older to have a multi-home run game in the World Series. Pearce was named World Series MVP. On November 16, 2018, Pearce re-signed for one year and $6.25 million. He was placed on the 10-day injured list prior to Opening Day of 2019, due to a left calf injury, then activated on April 4. Pearce batted .180 in 29 games through June 1, when he was placed on the injured list due to a low back strain. He was sent on a rehabilitation assignment with the Lowell Spinners on June 14, and with the Pawtucket Red Sox on June 18. His rehabilitation assignment was stopped by the team on July 1, for what was later described as a posterior ligament injury to his knee. Pearce was moved to the 60-day injured list on August 11, and did not play again during the season, finishing with one home run and nine RBIs in the 29 games he appeared in. He became a free agent on October 31. Pearce announced his retirement on April 13, 2020.  , or Retrosheet