Treanor, traded by the Rangers to the Royals for cash considerations at the end of Spring Training, was dealt back to the Rangers on Wednesday, also for cash. The deal leaves one opening on the Royals' 40-man roster.
Treanor has been on an injury rehabilitation assignment with Double-A Northwest Arkansas, the result of sustaining a concussion in a home-plate collision at Cleveland on July 30.
"He really gave us stability behind the plate," Royals manager Ned Yost said. "In Spring Training, we were trying to get through it with two young guys [Brayan Pena and Lucas May], and we saw that wasn't going to work. He came in and gave us veteran leadership, and a veteran presence behind the plate. He was fantastic."
The Rangers had to obtain Treanor by Wednesday in order for him to be eligible for postseason play.
Treanor played in 65 games for the Royals and batted .226 with three homers and 21 RBIs. He was on the Rangers' World Series team last season, batting .211 in 82 games. He played in four postseason games for Texas.
Previously, Treanor spent five years with the Florida Marlins and one year with the Detroit Tigers, missing most of the 2009 season with a hip injury.