Diego Castillo (born January 18, 1994) is a Dominican professional baseball pitcher for the Major League Baseball Seattle Mariners. He is 6 feet 3 inches tall.
He formerly played for the Tampa Bay Rays in Major League Baseball. 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 played the entire season, going 3–3 with a 3.96 ERA in 25 innings.
In 2015, he appeared in 18 bullpen appearances for both the Hudson Valley Renegades and the Bowling Green Hot Rods, collecting a combined 1–2 record and 3.03 ERA.
In 2016, he threw in the bullpen for both Bowling Green and the Charlotte Stone Crabs, recording a combined 3–6 record and 2.98 ERA in 60.1 innings thrown.
Castillo was demoted from the opener position in 2020. He took paternity leave on July 24. Castillo has four saves in 22 games, compiling a 1.66 ERA with 23 strikeouts and 11 walks.
During the postseason, he was routinely deployed in the late innings when the Rays were leading. He threw the final two innings of Game 5 of the American League Division Series against the New York Yankees, earning the victory after Mike Brosseau’s eighth-inning home run.