Pena was to join the Royals before Friday night's game against the Cleveland Indians. To make room on the active roster, Kansas City optioned left-handed pitcher Neal Musser to Triple-A Omaha.
The Royals will have to make another move on Saturday to clear a roster space for right-hander Kyle Davies, who will be recalled from Omaha to start against the Indians.
With Musser leaving, the Royals are down to 11 pitchers, and manager Trey Hillman said he wants to have a 12-pitcher staff. That means a position player will be dropped from the roster on Saturday.
General manager Dayton Moore knows Pena from their days together with the Braves. The catcher's arrival is not expected to affect the playing time of fellow backstops John Buck and Miguel Olivo.
"He's more of a long-term possible fit to our organization at that position. You look at our catchers, and Olivo is potentially a free agent at the end of the year," Moore said.
Pena has been strictly a pinch-hitter for the Braves this season, going 4-for-14 (.286), with one double. He has not caught or played any position on the field this year.
"He's a switch-hitter, a contact guy, more of a gap hitter. [He] can really receive, calls a game well, [has an] average arm; his release is pretty good. He runs well enough that we don't have to pinch-run for him," Moore said.
Pena has played in parts of four seasons for the Braves with a .228 average, two homers and 12 RBIs. In 40 games as a catcher, he's handled 137 chances without an error. He appeared in one game at third base and can also play the outfield.
Pena, 26, is from Havana, Cuba. In the Minor Leagues, he batted .303 with 15 home runs and 222 RBIs in 572 games. He is no relation to Royals shortstop Tony Pena Jr.
Musser's stay in Kansas City lasted just one day. He was recalled Thursday to replace an injured Leo Nunez and pitched one scoreless inning. Now, he's gone again.
To clear room on the 40-man roster for Brayan Pena, the Royals moved pitcher Luke Hudson from the 15-day to the 60-day disabled list.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.