The 31-year-old Soria was 3-1 with 23 saves and a 2.85 ERA in 43 appearances with Detroit this season. He didn't allow a run in 13 of his last 14 appearances. The deal was made a few hours after the Pirates got versatile reliever Joe Blanton from Kansas City for cash.
Soria will move into a setup role in a bullpen that already ranks among the league's best. Pittsburgh's 2.71 bullpen ERA ranked second to St. Louis for the NL lead.
"We feel it will just lengthen and deepen our bullpen," general manager Neal Huntington said.
Pittsburgh gave up infielder JaCoby Jones, a 23-year-old who was recently promoted to Double-A. The Pirates will have to make a move to create a roster spot when Soria joins the team, which opened a four-game series in Cincinnati on Thursday.
Kansas City reached the World Series last season with a deep bullpen that took a lot of the pressure off the starters. He said teams want to try to do the same.
"People are going to emulate success," Hurdle said. "The game seems to have gone in that trend."
Pittsburgh is second in the NL Central to St. Louis and leads the NL wild-card race.