On March 4, 2018, Brasier signed a minor-league contract with the Boston Red Sox. Pitching for the Pawtucket Red Sox of the Triple-A International League, he was selected to appear in the Triple-A All-Star Game. The Red Sox promoted Brasier to the major leagues on July 8; he made his Boston debut the next day, pitching one inning against the Texas Rangers and retiring the side in order. On August 30, Brasier recorded his first MLB win, pitching an inning of scoreless relief in a come-from-behind victory over the Chicago White Sox. Brasier proved to be a consistent reliever down the stretch, finishing with a 1.60 ERA in 34 appearances, and was the recipient of the Red Sox' Lou Gorman Award. Brasier was included on Boston's postseason roster, making a total of nine appearances and allowing one earned run in innings, as Boston went on to win the World Series.