Cousins, 27, was property of the Blue Jays for two weeks this offseason after Toronto claimed him off waivers from the Marlins on Oct. 17. But the Blue Jays designated him for assignment on Oct. 31 and the Mariners will now give him a look.
The 6-foot-1, 206-pounder joins the Mariners' outfield competition with returners Michael Saunders, Franklin Gutierrez, Casper Wells, Eric Thames, Mike Carp, Trayvon Robinson and Carlos Peguero. He does have one Minor League option remaining, while Carp, Robinson and Wells are out of options.
Cousins split last season between the Marlins and their Triple-A New Orleans club. He hit .163 in 53 games with the Marlins with four doubles, a triple, a home run and three RBIs in 86 at-bats while playing all three outfield positions.
Cousins hit .296 in 61 games for New Orleans with seven home runs, 36 RBIs and 14 stolen bases.
In three seasons with the Marlins, he batted .183 with a .231 on-base percentage and .291 slugging percentage over 128 games.
Cousins originally was selected by the Marlins out of the University of San Francisco in the third round of the 2006 First-Year Player Draft. He drew national attention in 2011 when his slide into home plate injured Giants catcher Buster Posey.
Cousins himself was hampered much of 2011 by back injuries unrelated to that collision and he played only 55 games that season. But he was healthy last season and got promoted to the Marlins in mid-June to fill a backup outfielder role.
The addition of Cousins puts the Mariners' 40-man roster at 37 players.