The Mariners optioned Montero to Triple-A Tacoma. He has 72 hours to join the club.
The 23-year-old Montero is hitting .208 with three home runs and nine RBIs. He's also been a designated hitter, and there are concerns about his impatience at the plate.
Seattle got Montero from the New York Yankees after the 2011 season. Montero made a nice impression in a brief call-up that year, hitting .328 with four homers and 12 RBIs, and the Yankees traded him and pitcher Hector Noesi to the Mariners for pitcher Michael Pineda and a minor leaguer.
Montero hit .260 with 15 homers and 62 RBIs last year in his first full season in the majors.
Montero had been splitting time with backup catcher Kelly Shoppach in recent weeks. The Mariners will make a corresponding roster move Friday -- their catchers in Triple-A are defensive-minded Jesus Sucre and touted prospect Mike Zunino.
Pineda, an All-Star as a rookie in 2011, has been injured and not pitched yet in the majors for the Yankees.
Pineda is currently pitching in extended spring training games and could be ready to start a minor league assignment after one more extended start early next week. He threw Thursday, which lines him to pitch again Tuesday.