Martin Gordon DeMerritt (born March 4, 1953) is an American professional baseball coach and a former minor league pitcher. In , he spent his sixth straight season as the pitching coach of the Rookie-level GCL Rays of the Gulf Coast League. He served two different terms in Major League Baseball as a coach for the San Francisco Giants (1989) and Chicago Cubs (1999).
Born in San Francisco, DeMerritt graduated from South San Francisco High School and was selected by the St. Louis Cardinals in the 22nd round of the 1971 Major League Baseball draft. The right-hander was listed at tall and . His active career, plagued by a sore arm, lasted six seasons (1971; 1973–77) in the Cardinals, Milwaukee Brewers and Houston Astros organizations, peaking at the Double-A level. Out of professional baseball at age 25, he worked in construction and as a bounty-hunter in California. He also coached in youth baseball in the San Francisco Bay area. In the early 1980s he was hired as a minor league pitching coach by his hometown Giants, and spent a brief period as a staff assistant with the MLB club in 1989, but eventually became the major league pitching coach for the Giants in 1989. He also coached in South Korea and in winter ball in Venezuela. DeMerritt was the first American to serve as a pitching coach in Korea.
He joined the Florida Marlins expansion team in 1992 as a pitching coach for their minor-league affiliates between 1992–94, before moving to the Cubs as a minor league coach (1995–98, including two years in Triple-A). He was the pitching coach of the MLB Cubs in on the staff of manager Jim Riggleman, but the Cubbies struggled to a 67–95 record with the pitching staff's 5.27 earned run average ranking 15th among the National League's 16 teams.
In 2001, the Tampa Bay Devil Rays hired DeMerritt as a pitching coach at the Class A level, and he has remained in the Rays' system since, working with pitchers at the lower levels of the minors for 18 seasons.