Montero, 24, was optioned to Triple-A Tacoma along with young outfielders Xavier Avery and James Jones. Those three players are all on the Mariners' 40-man roster.
The other player cut Friday was shortstop Chris Taylor, who isn't on the 40-man roster, so technically he was reassigned to Minor League camp. All four will continue working out in the adjacent Minor League complex and can still play in Cactus League games, if they're needed for that duty in the remaining 17 days of camp.
Montero was the Mariners' starting catcher at the start of 2013 after being acquired from the Yankees in a trade for Michael Pineda the year before, but struggled both with the bat and behind the plate and was sent down to Tacoma after hitting .208 in 29 games.
After playing 19 games in Triple-A last year, Montero injured his knee and then was hit with a 50-game suspension in the Biogenesis case. He came to camp this spring carrying extra weight, but has worked to lose that in the past few weeks and has hit well, batting .310 with three doubles, two home runs and five RBIs in 29 at-bats over 12 games.
First base hasn't gone nearly as smoothly, and the former catcher committed two errors in the same inning of Wednesday's game with the Cubs. He'll get a chance to work more on that in Tacoma.
Avery and Jones both played well this spring, but lost out in the numbers game in the outfield. The club still has seven outfielders in camp -- Dustin Ackley, Abraham Almonte, Michael Saunders, Corey Hart, Stefen Romero, Endy Chavez and Cole Gillespie, while utility man Willie Bloomquist and first baseman Logan Morrison can also play some outfield.
Jones hit .265 with a home run and six RBIs in 34 at-bats, while Avery batted .240 with five RBIs and three stolen bases in 25 at-bats.
Taylor, 23, was last year's Mariners Minor League Player of the Year after hitting .314 between Class A High Desert and Double-A Jackson, but didn't figure in the big league team's plans yet with Nick Franklin and Brad Miller competing for the shortstop job. He hit .158 with a double, a home run and five RBIs in 19 at-bats this spring.
The Mariners have 40 players remaining in their Major League camp, with 29 of those on the 40-man roster and the other 11 being non-roster invitees.