Wilhelmsen was available for Friday night's game against New York at Yankee Stadium. Manager Lloyd McClendon says Wilhelmsen will be used to bridge the gap to the back end of the bullpen.
The 31-year-old Wilhelmsen is 1-2 with a 4.68 ERA in 22 games for Seattle this season. He was sent down earlier this month after posting a 12.15 ERA in his last seven appearances with the Mariners.
Wilhelmsen pitched four innings for Tacoma without giving up a run after being sent down.
Wilhelmsen takes the roster spot left open when righty Danny Farquhar was sent to Triple-A on Sunday following the final game before the All-Star break.