Royals general manager Dayton Moore said the signing was to provide an option for additional depth, possibly as a left-handed bat off the bench.
"He's got a good track record and he's a guy with terrific makeup," Moore said.
The Royals are clinging to postseason hopes and Pena might help in the stretch run. Moore said the Royals will assess Pena, who hasn't played since being released, at Omaha and see where it goes. Major League rosters can be expanded on Sept. 1.
His best years were with the Tampa Bay Rays from 2007 to 2009 when he had 100 or more RBIs each season. In 2007, he had 49 homers, 121 RBIs and a .282 average.
Pena also played for Texas, Oakland, Detroit, Boston and the Chicago Cubs. In 1,471 Major League games, the left-handed batter has 285 homers, 816 RBIs and a .233 average. He was in the postseason twice with Tampa Bay.
He'll report to the Omaha Storm Chasers in Des Moines on Wednesday.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.