Byrnes, 33, is a career .260 hitter with 109 home runs, 396 RBIs and 128 steals in 948 games with four major-league clubs. Byrnes broke into the majors with the Oakland A's in 2000 and also played for the Colorado Rockies and Baltimore Orioles before joining the Arizona Diamondbacks for the past four seasons.
"We think Eric is a great fit for our team," Mariners Executive Vice President and General Manager Jack Zduriencik said. "He is a high-energy player with a veteran presence."
Byrnes has been limited by injuries the past two seasons, missing over two months with a fractured left hand in 2009 and playing in only 52 games in 2008 due to hamstring problems.