Stemle beat out Steve Andrade and Joel Peralta, who were designated for assignment and optioned to Triple-A, respectively.
Stemle, who yielded a solo homer to Eric Munson in the sixth inning during the Royals' 6-4 loss Saturday, finished the spring with a 5.00 ERA, having allowed five earned runs over nine innings. Andrade allowed two earned runs over 11 frames for a 1.64 ERA, while Peralta yielded four earned runs over 10 1/3 innings for a 3.48 ERA.
"It's the fact that we know him (Stemle) and J.P (Peralta) is still young and has got some options," Bell said. "This is more of we're sure of what we have in Steve. But we still really, really like J.P. It was not an easy decision."
Stemle joins Jeremy Affeldt, Denny Bautista, Ambiorix Burgos, Elmer Dessens, Scott Elarton, Jimmy Gobble, Joe Mays, Andrew Sisco, Luke Hudson and Mike Wood on an 11-man pitching staff.
The other roster positions are set: John Buck and Paul Bako are the catchers; Emil Brown, Shane Costa, David DeJesus and Reggie Sanders are the outfielders; and Angel Berroa, Esteban German, Tony Graffanino, Mark Grudzielanek, Doug Mientkiewicz, Matt Stairs, Mike Sweeney and Mark Teahen will be the infielders for Kansas City on Opening Day.
The Royals optioned right-hander Runelvys Hernandez to Triple-A, purchased the contract of right-hander Luke Hudson, placed right-hander Mike MacDougal and left-hander Mark Redman on the 15-day disabled list and placed Zack Greinke on the 60-day DL.
Alyson Footer is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.