Portis, 29, started all five games he played in an injury-riddled 2010, finishing with 227 rushing yards on 54 carries and two rushing touchdowns.
"Clinton provided excitement from the very first time he touched the ball as a Redskin and we were lucky to witness every ounce of energy, effort and passion he has given ever since," Redskins owner Daniel Snyder said in a statement.
"He will always be a Redskin and go down as one of the franchise's all-time greats."
In seven years in Washington, Portis rushed for 6,824 yards and 46 touchdowns, trailing only John Riggins (7,472) on the Redskins' rushing list.
"Being a Redskin was a special part of my life," he said. "Coming and being in that organization, I turned from a kid having fun to a man carrying responsibilities.
"I tried to put the world on my shoulders for (former) coach (Joe) Gibbs and the Redskins fans."
Portis, a second-round draft pick in 2002 by the Denver Broncos, ranks fourth among active players with 9,923 rushing yards in nine NFL seasons.
He was selected to the Pro Bowl for the 2003 season as a member of the Denver Broncos and the 2008 campaign with the Redskins.
(Reporting by Steve Ginsburg in Washington; Editing by Larry Fine)