Townsend's interception at chilly Heinz Field broke a 13-13 tie and capped a comeback that saw the home team trailing by 13 points in the fourth quarter.
The interception was the third of the game for the Pittsburgh (10-3) defense, which also notched three sacks and a fumble recovery.
Romo labored to complete 19-of-36 passes for 210 yards. He threw six completions to Jason Witten and three to Terrell Owens, who racked up 32 yards and a touchdown.
The brightest star for Dallas (8-5) was rookie Tashard Choice, who ran for 88 yards and caught five passes for 78 yards in his first start for the injured Marion Barber.
The Cowboys took a 13-6 lead on a 41-yard field goal with 7:15 to play in the game. The Steelers tied the contest when Ben Roethlisberger threw a 6-yard TD pass to Heath Miller with 2:04 to go.
Roethlisberger was 17-of-33 for 204 yards and the touchdown.