Gregory John Kosc (born April 27, 1949) 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.