The Redskins announced the deal Monday, their latest in a series of free agency moves for the defense. Johnson joins four other players brought in by new general manager Scot McCloughan for that side of the ball: cornerback Chris Culliver and linemen Terrance Knighton, Stephen Paea and Ricky Jean-Francois.
The Redskins have yet to sign a free agent for their offense.
Johnson was not drafted out of college at Boise State, joining the Seahawks when McCloughan worked for that team. Johnson was behind Kam Chancellor with Seattle and made one start at safety, getting more playing time on special teams.