Grant was signed last month after a pair of injuries left the backfield depleted behind starter Alfred Morris. The former Green Bay Packers back appeared in only one game with Washington, rushing once for five yards.
Williams was signed Tuesday. He started three games and rushed for 261 yards as an undrafted rookie with the Redskins in 2010. He spent last season and the first part of this season with the Detroit Lions before being released last week.
The Redskins also signed cornerback D.J. Johnson to the practice squad and released receiver Emmanuel Arceneaux from the practice squad.
Copyright 2012 by The Associated Press