Parker has been told there will be an open competition for the Redskins' starting job.
Parker, a Pro Bowler in 2006 and 2007, averaged 1,337 yards in three seasons for the Steelers from 2005 to 2007.
He became Pittsburgh's backup last season in his fifth full year as the Steelers turned to Rashard Mendenhall as their featured running back.
Parker finished 2009 with 389 yards on 98 carries.
Information from ESPN NFL Insider Adam Schefter was used in this report.