Early in the 2018 season, in an April 11 game against the New York Yankees at Fenway Park, Kelly hit Tyler Austin with a pitch, following a slide by Austin in which he was accused of spiking infielder Brock Holt. Austin charged Kelly and started a bench-clearing brawl; four players, including Kelly, were ejected. This was Kelly's first career ejection. On April 12, Kelly was suspended by MLB for six games and fined an undisclosed amount; he subsequently appealed, allowing him to continue playing until the appeal process completed. On April 26, Kelly's appeal was denied, meaning that his suspension would take effect from that point on. During his suspension, Kelly watched a game from the bleacher seats at Fenway Park. He appeared in a career high 73 games for the Red Sox, finishing with a 4.39 ERA in innings. In the postseason, Kelly made nine appearances, pitching innings while allowing one earned run and striking out 13. He was the winning pitcher, in relief, of Game 4 of the World Series, as the Red Sox defeated the Los Angeles Dodgers in five games.