Diego Castillo (born January 18, 1994) is a Dominican pitcher for the Seattle Mariners of Major League Baseball (MLB). He is 6 feet 3 inches tall. In Major League Baseball, he was a member of the Tampa Bay Rays.
Castillo signed as an international free agent with the Tampa Bay Rays in March 2014. That season, he made his professional debut with the Dominican Summer League Rays, where he pitched the entire season, going 3–3 with a 3.96 ERA in 25 innings.
In 2020, Castillo was moved away from the opener role. On July 24, he went on paternity leave. Castillo collected 4 saves over 22 games, posting an ERA of 1.66 with 23 strikeouts and 11 walks.
In the postseason, he was used in late innings consistently when the Rays had the lead. He pitched the final two innings of Game 5 of the ALDS against the New York Yankees, picking up the win after Mike Brosseau’s 8th inning home run. In the American League Championship Series against the Astros, Castillo appeared in 7 games, going 1–0 with 2 saves and 2 holds.
The first run he allowed was in Game 6. On July 29, 2021, the Rays traded Castillo to the Seattle Mariners for J. T. Chargois and Austin Shenton.