Brothers had been designated for assignment on Friday by the Rockies. The 27-year-old was a first-round Draft pick of the Rockies (34th overall) in 2009 and went 16-11 with a 3.42 ERA in 286 relief appearances over the past five seasons.
Brothers opened last season in Triple-A Albuquerque, going 5-2 with a 4.46 ERA, then was 1-0 with a 1.74 ERA in 10 1/3 innings over two stints with Colorado.
Greg Johns is a reporter for MLB.com. Follow him on Twitter @GregJohnsMLB, read his Mariners Musings blog, and listen to his podcast.