It was a wild game that turned three times in the final 5½ minutes and was far closer than most expected.
The Steelers struggled as Ben Roethlisberger lost two fumbles and threw an interception during an 11-minute first-half span, and they rushed for only 67 yards.
Even when they thought they had finally shaken off the feisty Colts, they hadn't.
After James Harrison forced Curtis Painter's fumble and Troy Polamalu ran it 16 yards for a 20-13 lead with 5:13 left, Painter led Indy on an 80-yard drive. It was capped by Joseph Addai's tying 6-yard TD run with 2:09 to go.
But Roethlisberger came back with a 60-yard march to set up the winning score.
The Colts stay winless on the season while the Steelers improved to 2-1 with the win.
Colts quarterback Kerry Collins is being evaluated for a possible concussion.
Collins had been knocked around for much of Sunday night's game against the Pittsburgh Steelers. He completed 13 of 29 passes for 93 yards before leaving the game. He started despite being hobbled in practice last week with a sore throwing shoulder.
Curtis Painter replaced him in the lineup.