Thomas, a fourth-round pick in the 2005 First-Year Player Draft, has only minimal big league experience so far in his career. The left-hander made eight appearances out of Seattle's bullpen in 2008, during which he allowed three runs on nine hits in four innings.
Thomas, 25, spent all of the '09 season as a reliever for the Mariners' Triple-A Tacoma (Wash.) affiliate. There, he finished 2-4 with a 4.48 ERA in 53 games. He walked 40 and struck out 53 in 60 1/3 innings pitched.
"Justin Thomas is a competitive, aggressive and physical left-hander with an average three-pitch mix," general manager Neal Huntington said. "With continued development, we feel he could be an interesting option for our bullpen."
As the Pirates look to fill current holes before the start of the 2010 season, finding some left-handed relief help is at the top of the list. Pittsburgh finished the season with two lefty relievers, but Donnie Veal is expected to begin the '10 as a starter in Triple-A and Phil Dumatrait struggled immensely after coming back from shoulder surgery.
With the addition of Thomas, the Pirates now have 39 players on the club's 40-man roster. Relievers Evan Meek and Jose Ascanio, both of whom are on still on the 60-day disabled list, still need to be added to that roster before Nov. 20, which means Pittsburgh has some roster shuffling yet to do.
Jenifer Langosch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.