Wainwright, 36, had been sidelined by a right elbow impingement. Manager Mike Matheny says Wainwright will pitch of the bullpen.
Matheny says the Cardinals will put Wainwright "in there when it works and see what we've got."
Wainwright pitched out of the bullpen when he made his big league debut with the Cardinals in 2005, but he hasn't made a relief appearance since the end of the 2015 season. He is 12-5 with a 5.12 ERA in 23 starts this year.
The Cardinals announced the move before Tuesday night's game at Cincinnati.