Roethlisberger went 24-of-33 through the air for 222 yards, sparking the Steelers (4-3) to their sixth straight victory at Heinz Field.
Jonathan Dwyer carried the load on the ground, racking up 107 yards on 17 carries.
The Redskins (3-5) were hampered by a slow day from Robert Griffin III, who managed to complete just 16-of-34 passes for 177 yards and one touchdown.
The rookie signal-caller was also stymied in his rushing game, gaining just 8 yards on six carries.
Alfred Morris picked up 59 yards on 13 catches for Washington.