Terms of the deal were not immediately available.
The Redskins had pursued Clark for the better part of three weeks after hosting him for a visit on March 13.
The Ravens and Seahawks also expressed interest in Clark, who opted to explore all of his options before settling on a team.
Clark, 34, spent the first two seasons of his 12-year career with the New York Giants, then played for Washington in 2004 and 2005. Washington let him depart via free agency in 2006, and he spent the past eight seasons with the Steelers, helping them win Super Bowl XLIII in 2009 and earning a Pro Bowl selection in 2011.
Safety is an area of need for Washington with unproven second-year players Phillip Thomas and Bacarri Rambo on the roster.
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