Starting catcher Russell Martin returned on Monday from a minor injury to his left knee, and while an MRI showed no structural damage, the Blue Jays have few options in the event Martin were to miss any time.
Kratz played 34 games with the Blue Jays in 2014, hitting .198 with three home runs and 10 RBIs before getting traded to the Royals in a midseason deal.
"Some depth, you know we know him," manager John Gibbons said. "He's played here before. He's a good defender."
Kratz also has a bit of experience catching veteran knuckleballer R.A. Dickey, having spent the majority of Spring Training in 2014 working with the former Cy Young Award winner. While Josh Thole is Dickey's personal catcher, Kratz provides an additional option in that area if needed.
"There's a benefit to that," Gibbons said. "When the [front office] made that move, they took that into consideration, no doubt."
Kratz split time earlier this season in a backup role with the Astros and Pirates, hitting .094 (8-for-85) with one home run and four RBIs prior to becoming a free agent.
Originally drafted by the Blue Jays in 2002, Kratz made his Major League debut with the Pirates in 2010. In his current go-around with the organization, Kratz will split time with prospect A.J. Jimenez in Triple-A Buffalo. The Bisons released Tony Sanchez to make room on the roster.