Ryan, 56, became known for his long hair and intense personality with players and officials during his run as a coordinator in Oakland, Cleveland, Dallas and New Orleans.
He is entering his 20th NFL season, though he last coached in 2016 as Buffalo's assistant head coach/defense.
During his run with the Saints (2013-15), Ryan led a defense that ranked fourth in the league in points allowed and second in passing defense.
He will be working under head coach Jay Gruden and defensive coordinator Greg Manusky in Washington.
The Redskins finished 7-9 this past season and missed the playoffs. They were 15th in points allowed and 16th in yards per game, their best defensive numbers since 2009.