Sucre, 25, has been in the Mariners' Minor League system the past two years. It's expected he'll join the club in time for the start of a three-game series at Safeco Field on Friday night against the Rangers and work as the backup to veteran Kelly Shoppach.
Montero, 23, is batting .208 with three home runs and nine RBIs in 101 at-bats over 29 games this season. He has struggled both at the plate and defensively in his second season with the club after being obtained from the Yankees for right-handed pitcher Michael Pineda last year.
Montero, one of the top-rated prospects in baseball when he was coming up with the Yankees, hit .260 with 15 home runs and 62 RBIs last year with Seattle while splitting time between catcher and designated hitter. He was given the starting catching job this season by the Mariners, but has split time recently with Shoppach.
By rule, Montero will have 72 hours to report to Tacoma.
Sucre is hitting .302 (16-for-53) in 14 games for Tacoma, where he's split time with prospect Mike Zunino. Sucre went 3-for-4 with a two-run double in Wednesday's 5-4 win over Nashville.
Considered a strong defensive catcher, Sucre spent last year with Double-A Jackson, where he hit .271 in 90 games after signing with the Mariners as a Minor League free agent.
Sucre originally signed by the Braves in 2005 as an international free agent out of Venezuela. He has thrown out seven of 18 attempted basestealers this season and has not committed an error in 110 consecutive games, dating back to Aug. 27, 2011.
In nine Minor League seasons, Sucre has hit .253 with 21 home runs and 176 RBIs in 497 games while throwing out 42.4 percent of attempted basestealers.
Zunino, 22, is the Mariners' top catching prospect after being drafted with the third pick in the first round last June out of Florida. But he's played only 77 Minor League games so far, including 33 this year with Tacoma. After a hot start, Zunino is hitting .220 with seven home runs and 35 RBIs for the Rainiers.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB as well as his Mariners Musings blog. This story was not subject to the approval of Major League Baseball or its clubs.