Pena, 36, hit .207/.321/.346 (85 OPS+) with 13 doubles, eight homers and 25 RBI in 89 games for the Astros and Royals last season. The former All-Star averaged 39 homers and 108 RBI per season for the Rays from 2007-09 but contact issues and a low batting average caught up to him in recent years.
Pena was with the Angels this past spring, but he was released in late March.
This move still has the potential to impact the majors, though, because of the Rangers injury woes at first base. Both Prince Fielder and Mitch Moreland have had major surgery and are expected to miss the remainder of the season. Career minor-leaguer Brad Snyder has been getting the starts at first the past few games and the top corner infield prospect in the system, Joey Gallo, was just recently promoted to Double-A.
Pena will have to prove himself at Triple-A, but if he does so, he might well be back in the majors this season.