Swanson was recalled from Triple-A Tacoma prior to Tuesday's game against the Royals, Greg Johns of MLB.com reports.
Felix Hernandez (illness) only logged one inning in Monday's start, so the Mariners are bringing up Swanson to add some length to a depleted bullpen. He could be used as a starter in the majors at some point this season, and will likely be sent back to Triple-A to continue developing as a starter after he has served his purpose as short-term relief help.