Camp, 36, pitched the past four seasons for the Blue Jays and signed a one-year deal as a free agent with the Mariners in February.
Camp pitched well this spring, with a 2.16 ERA in 8 1/3 innings in seven games, but manager Eric Wedge said the move was a result of the development of some younger prospects.
"We expected him to be on our ballclub when we made that decision this winter," Wedge said. "But when you look at some of these young kids and how they performed and how they're going to be in this mix -- and quite frankly, a few of them will be on our club -- that was really the separator.
"Shawn is going to be in the big leagues with somebody. But ultimately when you talk about the options and depth we have and the way these kids have stepped up a little quicker than we anticipated, that's where we are."
The decision to put Japanese starter Hisashi Iwakuma in the bullpen as a long reliever eliminated one opening. The club is also taking a long look at Rule 5 Draft pick Lucas Luetge, as well as 21-year-old rookie Erasmo Ramirez.
The Mariners must reduce their roster to 30 players prior to heading to Japan on Thursday for a week-long trip that will include two exhibition games and two regular-season contests against the A's on March 28-29.
The release of Camp opens a spot on the 40-man roster, which is now at 38 players.
Camp has a 30-28 record and 3.55 ERA in eight Major League seasons, having pitched with the Royals, Rays and Blue Jays.