John Reilly Costello (born December 24, 1960 in the Bronx, NYC, NY) is a former relief pitcher in Major League Baseball who played for the St. Louis Cardinals (1988-1990), Montreal Expos (1990) and San Diego Padres (1991). He batted and threw right-handed.

John was signed as a 25th round pick in the 1983 amateur draft by the St. Louis Cardinals and scout Tim Thompson. Costello was 27 years old when he broke into the big leagues on June 2, 1988, with the St. Louis Cardinals. He had two very good seasons in the St. Louis Cardinals bullpen, going 5-2, 1.81 with 1 save in 36 games in 1988 and 5-4, 3.32 with 3 saves in 48 games in 1989. After that, he imploded, making 8 appearances to sky high ERAs with St. Louis and the Montreal Expos in 1990, making a final 27 appearances with the San Diego Padres in 1991.

Costello is featured briefly in Steve Fireovid's book "The 26th Man"; during Steve's 1990 season with the Indianapolis Indians (Triple A affiliate of the Expos), John was given big league opportunities Steve and his teammates felt were unwarranted due to his poor number, however his success in the majors explains why the big league club was willing to give him opportunities denied to others.

In a four-season career, Costello posted an 11-6 record with a 2.97 ERA and four saves in 119 games pitched.

(excerpted from Baseball Almanac, BR Bullpen & Wikipedia)