John Clark Nelson (born March 3, 1979 in Denton, Texas) is a former Major League Baseball shortstop with the St. Louis Cardinals. Drafted out of the University of Kansas, Nelson was originally an outfielder, but the St. Louis Cardinals converted him to a shortstop.
Nelson was promoted to the Cardinals on September 5, 2006, wearing the number 62, and made his major league debut on September 7, striking out against the Arizona Diamondbacks' Greg Aquino. In his first big-league action he went 0-5 with 4 strikeouts. He also pinch-ran 3 times and scored 2 runs, playing one game at shortstop and one game at first base.
In 2008, he signed with the Lancaster Barnstormers of the Atlantic League of Professional Baseball.