Roethlisberger, intercepted three times in the first half, threw for 272 yards -- 224 in the fourth quarter -- as the Steelers (4-6) scored 21 points in the final period, avoiding a defeat that would have made it almost impossible to get into the playoffs.
"People put us down and out, but we're not going to quit," said Roethlisberger, who brought Pittsburgh back a week ago to beat New Orleans and shook off a sore shoulder in the fourth period. "We stayed together and came back to win."
"I scrambled and saw Willie," said Roethlisberger, 18-for-29 in the final 15 minutes. "I told him later I thought about giving it to him. He didn't like that, but I was joking. I knew he'd get in."
Parker, whose 1-yard TD run with 4:14 left brought the Steelers within 20-17, gave all the credit to his quarterback.
"It was completely improvised by Ben," Parker said. "It was a great play. He's our leader. We came back when he did it all."
The Browns (3-7) spent all week talking about getting revenge, and for most of the rainy afternoon it looked like they would get it.
But Cleveland's defense couldn't stop Roethlisberger when it mattered most, and the Browns dropped their sixth straight game to their bitter rivals, who have won 12 of the last 13 between the teams.
"We had them," said tight end Kellen Winslow, who engaged in some serious pregame trash talking with Pittsburgh's Joey Porter. "We're in every single game. We just have to find a way to pull it out at the end."
Porter had no sympathy.
"If you talk and lose, you're still losers," he said. "They talk like a different team, but in reality they are not. They had all the breaks, all the bounces. But when it was nail-biting time and you've got to make plays, who made them?
After Roethlisberger's hookup with Parker, the Browns drove to Pittsburgh's 22. With 3 seconds left, Charlie Frye threw a jump ball to the back of the end zone that Braylon Edwards nearly grabbed in a crowd.
But when the ball fell to the ground, several of the Steelers dropped to their knees in relief. Edwards, Winslow and other Cleveland players consoled each other after another tough loss.
"This is very disappointing, especially after that 41-0 loss last year," linebacker Andra Davis said. "We've been looking forward to this one since that clock went to zeroes."
Finally finding his touch, he completed three third-down passes as Pittsburgh drove 79 yards and pulled within 20-17 on Parker's 1-yard run.
The Steelers got the ball back with 3:06 left, and Roethlisberger hit passes of 13, 15 and 21 yards to get Pittsburgh in range for a possible tying field goal. He then hooked up with Holmes for a first down at Cleveland's 16. Two plays later, he bought himself just enough time from Cleveland's rush to flip the ball to Parker.
"Ben's got eyes to find you, so you better stay ready all the time," Parker said. "He'll make something out of nothing. And he got us this win."
Roethlisberger finished 25-of-44 for 272 yards and two TDs. Frye was 17-of-27 for 224 yards.
Cleveland's offense wasted two scoring chances in the first quarter, but the defense gave the Browns a 7-0 lead as cornerback Daven Holly returned a tipped pass 57 yards to make it 7-0.