Gregory John Kosc (born April 27, 1949 in Bridgeport, Connecticut) is a former umpire in Major League Baseball who worked in the American League from 1976 to 1999. He officiated in the World Series in 1987 and 1997, and in the All-Star Game in 1981 and 1992. He also worked the American League Championship Series in 1979, 1988 and 1993, and the American League Division Series in 1996 and 1997. Kosc wore uniform number 18 when the American League umpires adopted them in 1980.
Kosc was the first base umpire for Len Barker's perfect game on May 15, 1981, and was behind the plate for Mike Witt's perfect game on September 30, 1984; he is one of only seven umpires to have worked in two perfect games. He was an umpire on September 6, 1995 when Cal Ripken Jr. broke Lou Gehrig's consecutive games played streak. He was also one of the umpires for the single-game playoff to decide the AL West title in 1995. He was one of 22 umpires who submitted their resignations as part of a failed union strategy in 1999, a move that backfired when Major League Baseball opted to accept the resignations. He is a resident of Medina, Ohio.
- The Official Major League Baseball Fact Book. The Sporting News. 2002. p. 503.
- article on Kosc's last gameESPN.com1999