Flacco threw two touchdown passes to wide receiver Mike Wallace and Baltimore, sparked by returning a blocked extra point, rallied from 20 down and beat the Cleveland Browns 25-20 on Sunday.
Justin Tucker kicked three field goals - the last with 2:56 left - and the Ravens (2-0), who were down by nearly three touchdowns less than 10 minutes into the game, withstood Cleveland's final drive to improve to 15-2 against the Browns under coach John Harbaugh.
In his second game back after undergoing knee surgery that ended his 2015 season, Flacco again showed why he's one of the NFL's top quarterbacks and one of the game's most unflappable leaders.
"I've probably never been around someone as cool as Joe," said Wallace, who signed as a free agent this winter. "Nothing ever bothers him."
The Ravens scored 25 unanswered points and hung on when linebacker C.J. Mosley intercepted Browns quarterbackJosh McCown at the goal line with 13 seconds left.
"It was a good way to end it, on defense," Mosley said. "We just had to pull it out."
Baltimore, which went an uncharacteristic 5-11 last season, was in danger of getting blown out after McCown's second TD pass to rookie Corey Coleman in the first quarter made it 20-0.
But Ravens defensive end Lawrence Guy got a hand on Patrick Murray's extra point and rookie cornerback Tavon Young scooped up the bouncing ball and returned it for a rare, two-point defensive conversion - a play that ignited the comeback.
"Nobody cracked. Nobody panicked," Harbaugh said. "Nobody pointed a finger."
It was the second-biggest comeback in Baltimore's history, and Flacco told his teammates afterward he was proud to be part of it.
"It wasn't pretty, it wasn't great," he said. "Do you want to play that kind of game in September? Not necessarily, but every single game in January and come February, where we hope to be, is going to be like that."
McCown stayed in the game despite an injured left shoulder for the Browns (0-2), who were hurt by a missed field goal and three costly penalties in the fourth quarter.
Cleveland was driving for a go-ahead TD in the final minute behind McCown, who completed a 20-yard pass to Terrelle Pryor to the Baltimore 10 with 27 seconds left.
However, the big play was nullified because Pryor, who was held on the play by Webb, was called for taunting when he flipped the ball toward the Ravens defensive back.
Webb said he wasn't sure if Pryor said anything.
"If he said some things, I guess the referee heard it," Webb said. "They are real tough on those things now. Good call, that's what I'm going to say."
Pryor felt he did nothing wrong.
"I'm just saying there's other people that can catch a ball and spin it and look at players in the face," Pryor said. "But if I get up and drop the ball, clearly I wasn't trying to drop it on nobody."
HEAVY HEARTS: The Ravens dedicated the win to defensive line coach Clarence Brooks, who passed away Saturday at 65 from cancer.
Brooks joined Baltimore's staff in 2005 and helped the Ravens develop one of the NFL's top defensive fronts. He spent time with the team during training camp.
"That was for Clarence Brooks," Harbaugh said. "I think what the players did out there in his name says it all."
HURTING QB: Filling in for Robert Griffin III who broke a shoulder bone in last week's opener, McCown showed amazing heart in playing hurt.
Following the game, McCown couldn't lift his left arm and his eyes welled with tears as he talked about staying on the field. He said his "plan" is to play next week against Miami, but the Browns may have to turn to rookie Cody Kessler.
"I'd go to war with that guy anytime," Browns guard John Greco said of McCown. "That guy gets the (stuff) knocked out of him seven, eight times a game, keeps getting up and making throws."
STUNNING START: Nobody expected the Browns to do what they did in the first 15 minutes, which ended with them leading 20-2.
It was the team's largest lead after one quarter since Dec. 3, 1961, when the Browns led Dallas 21-0 en route to a 38-17 win.
BLOCK PARTY: Down 20-0 and needing something, anything, to get them going, the Ravens executed the blocked extra point, which Harbaugh said embodied his team's fight.
"It's the toughest two seconds in football," he said. "Then a rookie (Young) is exactly where he's supposed to be in case it gets blocked, he's right there to scoop and score. That's the kind of thing as a coach you feel really good about."
AS THE CROW FLIES: Isaiah Crowell's 85-yard TD run helped the Browns back finish with 133 overall. Taking a handoff up the middle, Crowell got an excellent block from fullback Malcolm Johnson, blasted through the secondary and went untouched for the second-longest TD run in Cleveland history. Bobby Mitchell ripped off a 90-yarder in 1959.