John Reilly Costello (born December 24, 1960) 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.
Costello graduated from Oceanside High School in Oceanside, New York in 1979 and then played college baseball at Mercyhurst where he led all of NCAA with a 1.12 earned run average in 1982.
In a four-season career, Costello posted an 11–6 record with a 2.97 earned run average and four saves in 119 games pitched.