Timlin made 47 appearances during the 2008 season, with a record of 4–4, 1 save, and a 5.66 ERA (the highest of his career). He also made five rehabilitation appearances with Triple-A Pawtucket. In the postseason, he appeared in two games against the Tampa Bay Rays in the ALCS; he took the loss in Game 2 after giving up a run in the 11th inning, and his two innings in relief during Game 4, which Boston lost by nine runs, would be the final MLB appearance of his career. After the season ended, Timlin again became a free agent.