Adams, 25, played in every game last season and started twice. He registered 30 tackles last season.
He has 66 tackles and three interceptions this season.
He's going to be a punt returner for the Seahawks and compete for playing time at cornerback.
Adams is a 5-foot-11, 195-pound former San Francisco 49ers seventh-round draft pick.
He was with the Seahawks in 2011 before being cut in 2012 and has been playing for the Raiders for the past two seasons.
He has a previous stint with the New England Patriots.
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Aaron Wilson covers the Ravens for The Baltimore Sun