Criswell signed a one-year, $800,000 contract with the Red Sox on Thursday, Robert Murray of FanSided.com reports.
Criswell appeared in seven games for Boston in 2025, finishing the regular season with a 3.57 ERA and 1.58 WHIP through 17.2 innings. He put up similar numbers at Triple-A Worcester (3.70 ERA, 1.35 WHIP) before elbow inflammation put an early end to his season in late August. The 29-year-old righty doesn't have any more minor-league options remaining, so he will have to make the Red Sox's Opening Day roster to avoid losing his 40-man spot.