Diego Castillo (born January 18, 1994) is a Dominican professional baseball pitcher in the Colorado Rockies organization.

He has previously played in Major League Baseball (MLB) for the Tampa Bay Rays, Seattle Mariners, and Minnesota Twins.

Castillo signed with the Tampa Bay Rays as an international free agent on March 5, 2014.

In 2015, he played for both the Hudson Valley Renegades and Bowling Green Hot Rods, compiling a combined 1–2 record and 3.03 ERA in 18 relief appearances.

He was named the Rays' minor league reliever of the year after posting a 2.76 earned run average (ERA) with 90 strikeouts and 15 saves in 71.2 innings pitched.

[6] In the postseason, Castillo threw 2 shutout innings in the American League Wild Card victory the Oakland Athletics.

In the American League Championship Series against the Astros, Castillo appeared in 7 games, going 1–0 with 2 saves and 2 holds.

[7] On July 7, 2021, Castillo combined with Collin McHugh, Josh Fleming, Matt Wisler, and Peter Fairbanks to no–hit the Cleveland Indians.

[9] He made 24 appearances for Seattle down the stretch, registering a 3-1 record and 2.86 ERA with 26 strikeouts and 2 saves in 22.0 innings pitched.

In 2022, Castillo made 59 appearances for Seattle, logging a 7-3 record and 3.64 ERA with 53 strikeouts and 7 saves in 54.1 innings of work.