The 31-year-old right-hander, who began this season with the New York Mets, tested positive for an amphetamine -- considered a performance-enhancing substance -- while pitching for Triple-A Tacoma in the Pacific Coast League.
The suspension is effective immediately.
Seattle signed Sosa to a minor league contract in July after he was designated for assignment by the Mets on May 13.
Sosa was 4-1 with a 4.10 ERA in 20 games for the Mets this season after going 9-8 with New York in 2007. After making his debut with Tampa Bay in 2002, Sosa also had stints with Atlanta and St. Louis in seven major league seasons.
His career mark is 40-49 with five saves and a 4.67 ERA in 254 major league appearances, including 88 starts.