Mott has pitched 22 innings in 24 appearances this season. He has a 4.91 ERA.
The move, announced Friday, is retroactive to Aug. 1.
The 32-year-old Motte underwent elbow ligament surgery in May 2013 and started this season on the DL. He made his 2014 debut May 21.
To fill Motte's place on the roster, the Cardinals recalled lefty reliever Nick Greenwood from Triple-A Memphis.
Greenwood made his major league debut earlier this season. From June 15-July 22 he threw 19 innings, including a four-inning stint against the Los Angeles Dodgers.
He was expected to join the team for Friday night's game against the Baltimore Orioles.