John Stephen "Thumper" DeMerit (January 8, 1936 in West Bend, Wisconsin) is a former professional baseball player. He was an outfielder over parts of five seasons (1957–1959, 1961–1962) with the Milwaukee Braves and New York Mets.
DeMerit was a member of the 1957 World Series champion Braves. He also appeared in 14 games for the 1962 Mets, whose 120 losses continue to be the most by a Major League team in a single season since the 19th Century.
An alumnus of the University of Wisconsin–Madison, for his career he hit .174 with 3 home runs and 7 runs batted in in 93 games played.