The Seahawks announced the signing on their website Monday.
The 27-year-old Swain was drafted by Green Bay in 2008, taken in the seventh-round out of San Diego State. He caught six passes with Green Bay in the 2010 season and another two for San Francisco in 2011, but was among the 49ers' final cuts before the start of last season.
Swain may have a tough time making the Seahawks roster with the addition of Percy Harvin this offseason to a receiver corps that also includes Sidney Rice, Golden Tate and Doug Baldwin.
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