The Mariners optioned de Geus to Triple-A Tacoma on Monday.
The Mariners selected de Geus' contract from Triple-A Tacoma on Monday, Ryan Divish of The Seattle Times reports.
De Geus signed a minor-league contract with the Mariners on Sunday.
De Geus has signed with the yet-to-be-named Atlantic League based in Frederick, Maryland.
De Geus was designated for assignment by the Diamondbacks on Friday.
De Geus (3-2) allowed a run and struck out one over a scoreless sixth inning to pick up the win over Pittsburgh on Wednesday.
De Geus (2-0) earned the win Wednesday versus Pittsburgh. He allowed an inherited runner to score on a walk, taking a blown save in two-thirds of an inning.
De Geus was recalled from Triple-A Reno on Saturday.
The Diamondbacks claimed de Geus off waivers from the Rangers on Friday.
The Rangers designated de Geus for assignment Wednesday.