Johnathan Keith Gainer (born October 8, 1966) is an American former Major League Baseball first baseman who appeared in 23 games for the Colorado Rockies in its 1993 season.

A member of the inaugural Rockies team, Gainer was previously selected by the San Diego Padres in the 24th round of the 1990 MLB Draft.

Gainer would make his Major League Baseball debut with the Rockies on May 14, 1993 as their starting first baseman and wore #37.

Afterwards, Gainer appeared in his final game on October 2 of that year as a pinch-hitter for Lance Painter.

Overall, he hit .275 with 121 home runs and 484 RBI in 11 minor league seasons spanning 1990–2001.