Six Steelers recorded a sack as the NFL's top-ranked defense held Washington to just 13 first downs and 221 total net yards.
Byron Leftwich, who came on in relief of injured Pittsburgh starting quarterback Ben Roethlisberger, completed 7-of-10 passes for 129 yards and a touchdown. Roethlisberger exited the game at halftime with a right shoulder injury.
Roethlisberger connected on just 5-of-17 passes for 50 yards with an interception and a 1-yard touchdown run before getting hurt.
"Potentially he was capable of going back in the game," Steelers Coach Mike Tomlin said of Roethlisberger. "But we went on the field with Leftwich to start the second half. We'll see how he is in the morning."
Willie Parker ran for 70 yards and a touchdown for the Steelers. Santonio Holmes caught three passes for 30 yards and a score for Pittsburgh (6-2), which took a one-game edge over Baltimore in the AFC North.
Jason Campbell completed 24-of-43 passes for 206 yards and two interceptions as Washington fell to 6-3. The NFL's leading rusher, Clinton Portis, managed just 51 yards.
Before this game, Campbell hadn't thrown an interception in 230 attempts this season.