On February 26, 2018, Martinez signed a five-year, $110 million contract with the Boston Red Sox. On April 7, Martinez hit his first home run in a Red Sox uniform at Fenway Park off Chaz Roe of the Tampa Bay Rays. Martinez won the AL Player of the Week Award for the week ending on May 20, after hitting five home runs, eight RBI, .346 average, and 1.414 OPS in seven games. At that point, he was tied with teammate Mookie Betts for the major league home run lead at 15, making them the first duo in Red Sox history to both hit at least 15 home runs within the first 50 games of the season. Batting .329 with 27 home runs and 73 RBIs, Martinez was named to the 2018 MLB All-Star Game as the American League's starting designated hitter. Spanning approximately one full season of play from the 2017 MLB All-Star Game to July 14, 2018, he led the major leagues in home runs (60), RBIs (152), and OPS (1.074). In a 19-12 win over the Baltimore Orioles on August 10, Martinez became the first player to reach 100 RBIs in the 2018 MLB season. Martinez again won the AL Player of the Week Award, for the week ending August 12, when he hit .464 with 11 RBIs and nine extra-base hits. He was also recognized as the AL Player of the Month for August, with a slash line of .373/.453/.686 with seven home runs and 25 RBIs in 26 games. Martinez finished the 2018 season with 43 home runs and a career high 130 RBIs, which led MLB. He led the Red Sox in home runs, RBIs, and hits. In Game 1 of the ALDS, Martinez hit a three-run home run in the first inning off of New York starter J. A. Happ; Boston would win the game 5-4. In Game 5 of the ALCS, Martinez hit a solo home run off of Houston starter Justin Verlander; Boston won the game, 4-1, and clinched a berth in the World Series. On October 26, Martinez was announced as the American League recipient of the annual Hank Aaron Award. He also became the first player in history to win two Silver Slugger Awards in the same season: one as an outfielder and one as a designated hitter. In Game 5 of the 2018 World Series, Martinez hit a home run in the seventh inning to increase the Red Sox lead to 4-1. The Red Sox ended up winning the game and their ninth World Series title.