The 36-year-old pitcher was in camp as a non-roster player, trying to make a comeback after sitting out last season. The Cubs informed him Friday that he would not make the team out of spring training, so Looper decided to retire.
"After taking a year off, he came in and gave it a good shot," Cubs general manager Jim Hendry said. "We just felt in the end he wasn't going to be able to break with the 12 [pitchers]. He certainly had an outstanding career. We felt it was a good, sound idea. I'm glad we did it. He handled himself like a true professional and was very helpful to the young pitchers in camp and was very appreciative of the opportunity and certainly understood why he could not break with the 12."
Looper did not pitch last season after going 14-7 with a 5.22 ERA in 2009 for the Milwaukee Brewers. He made his major league debut with the St. Louis Cardinals in 1998.
The right-hander, drafted No. 3 overall by St. Louis in 1996, spent the first nine seasons of his big league career as a reliever before working as a starter from 2007-09. He finished 72-65 with a 4.15 ERA and 103 saves.