The sixth-seeded Ravens (13-6) rebounded from a 5-11 season because of the big arm and poise of Flacco. But they couldn't return to the Super Bowl in Tampa, Fla., because of him.
With a windchill of 15 degrees at Heinz Field, the NFL's top-ranked defense turned Joe Cool into Joe Cold.
Pressured throughout the game, Flacco completed 43.3 percent of his passes (13 of 30) for 141 yards, tying a career worst with three interceptions (after not throwing an interception in his first 49 playoff passes). His quarterback rating of 18.2 was his lowest in 19 NFL games.
Adding injury to insult, Ravens running back Willis McGahee had to be stabilized and carted off the field after a violent hit by Steelers safety Ryan Clark in the final four minutes.
McGahee took a short pass from Flacco, turned to go upfield and ducked just as he was about to be hit by Clark. He fumbled on the hit, and the ball was recovered by the Steelers.
McGahee had movement in his arms and legs but had significant neck pain, according to a report from the Ravens' locker room. The team later said McGahee is "neurologically intact," but would be spending the night in a Pittsburgh hospital.
"We'll move on," Flacco said. "But we're a little disappointed at this point."
Behind 16-14 in the fourth quarter, Flacco had a chance to win the game. Instead, the normally calm quarterback lost it.
That sealed the Ravens' eighth loss in their past nine trips to Heinz and propelled the second-seeded Steelers (14-4) to the Super Bowl, where they will play the Arizona Cardinals.
"I think Troy was able to read my eyes," Flacco said. "He was able to jump over there and make a play. I didn't see him over there until I was on the ground."
The mistake epitomized the rockiest game for Flacco, who had become the first rookie quarterback to win two playoff games.
The consensus was that this loss was a setback but would not derail Flacco, whose play revitalized an organization that has long searched for a franchise quarterback.
"I'm not going to sit here and say Joe played a certain way," first-year Ravens coach John Harbaugh said. "Joe went out there and competed, battled, fought and tried to find a way to win a football game. I don't have any complaints about that."
The Ravens' defense, which had thrived on takeaways during the regular season and playoffs, caused only one yesterday - a fumble forced by Ray Lewis.
Still, the NFL's second-ranked defense had only one major blunder, allowing a 65-yard touchdown pass from Ben Roethlisberger to Santonio Holmes. For the rest of the game, the Ravens limited the Steelers' offense to three field goals (34, 42 and 46 yards).
The real problem was a long-standing one for the Ravens - an inconsistent offense - which resurfaced in the team's first championship game since January 2001.
The Ravens didn't produce a first down until the second quarter. They converted on only three of 13 third-down opportunities (23 percent).
And both of their scores were helped by special teams - a long punt return by Jim Leonhard and a poor punt by the Steelers - and set up by pass-interference penalties.
Still, the defense refused to point fingers at either the offense or Flacco, who has been roughed up by the Steelers in three losses to Pittsburgh this season.
"I didn't think he struggled," linebacker Terrell Suggs said. "They just made plays. It was their night. He was getting hit when he was throwing. He made the AFC championship in his first year. The only thing is up for him."
Flacco, though, never could get in a rhythm because of the Steelers' relentless pass rush and his inaccurate passing.
When Holmes' long touchdown put the Ravens into a 13-0 hole, Flacco had misfired on seven of his first eight passes.
At that point, the Ravens hadn't gained a first down and were out-gained 156-17.
The Ravens' offense got a break near the end of the second half with special teams and a questionable call by the officials.
Leonhard's 45-yard punt return set up the Ravens at the Steelers' 17-yard line. On third-and-nine, Steelers cornerback Bryant McFadden appeared to break up a pass to Mason cleanly, but the officials flagged him for pass interference.
Then McGahee scored on a 3-yard run - his first postseason touchdown - to cut the deficit to 13-7.
The Ravens inched closer in the fourth quarter when Pittsburgh's Mitch Berger shanked a 21-yard punt. A second pass-interference penalty by the Steelers - this time by cornerback Ike Taylor on rookie wide receiver Marcus Smith in the end zone - set up another Ravens score.
McGahee ran in from 1 yard out to close the score to 16-14 with 9:29 left in the game.
But turnovers on their last three possessions (two interceptions by Flacco and a fumble by McGahee) ended any hopes of a comeback.
Suggs said the loss has yet to set in with the players.
"We're still feeling a little shocked and disappointed," he said.
After the Ravens fell short of reaching the Super Bowl, Harbaugh remained upbeat in his final message to the team.
"This is our beginning. This is where we start," he said. "This is not an end by any stretch. We're excited to go forward and take the next step."