Wittgren signed a contract with the Mariners on Sunday, Jon Heyman of the New York Post reports.
Wittgren is expected to elect free agency after being sent to Triple-A Omaha on Wednesday, Anne Rogers of MLB.com reports.
Kansas City optioned Wittgren to Triple-A Omaha on Wednesday.
The Royals selected Wittgren's contract from Triple-A Omaha on Tuesday.
Wittgren was reassigned Tuesday to the minor-league side of Royals camp.
Wittgren signed a minor-league contract with the Royals on Monday.
The Cardinals granted Wittgren his release Monday, Derrick Goold of the St. Louis Post-Dispatch reports.
Wittgren was designated for assignment by the Cardinals on Saturday.
Wittgren (1-0) allowed a hit in a scoreless inning to earn the win Saturday over the Reds.
Wittgren recorded the last out of Wednesday's 5-2 win over the Padres, earning a save.