Dustin Hopkins kicked a 40-yard game-winning field goal as time expired to reinsert the 6-3 Browns into the AFC North race. Cleveland, which has not won a division title since 1989, now trails 7-3 Baltimore by just a half-game.
The Browns -- who had not won a game after trailing by two touchdowns in the fourth quarter in nine years -- became just the fourth team in the Super Bowl era to win after trailing in the first minute and not leading until the final minute of regulation.
"We didn't back down," Browns quarterback Deshaun Watson said. "Definitely a big moment for us."
A big moment for Watson, especially.
Watson threw a pick-six to Kyle Hamilton on the game's opening series and completed just 6 of 20 passes in the first half as the Browns trailed 17-9.
But he responded after halftime despite battling a left ankle injury.
Watson completed all 14 of his second-half attempts for 134 yards and a touchdown, and guided the Browns down the field on the final possession to set up the winning field goal. On that drive, he completed all three of his passes for 30 yards and rushed 16 yards for a first down.
"He's a warrior," Browns coach Kevin Stefanski said. "Makes plays when they're not there. Makes unbelievable throws. Never a doubt in his mind how this game was going to turn out."
Cleveland faced its biggest deficit, 24-9, after Odell Beckham Jr.'s 40-yard touchdown catch to open the second half. But Watson and the Browns came back with a 17-play, 10-minute touchdown drive, capped by the quarterback's 2-point conversion run.
After another Ravens touchdown, Watson answered again in the fourth quarter with a 10-yard, scrambling TD toss to Elijah Moore.
"He's a gamer," Browns All-Pro guard Joel Bitonio said of Watson. "All week I sense this focus from him."
The Browns trailed for 59 minutes and 20 seconds. No NFL team this millennium had won while trailing for that long in a game, according to ESPN Stats & Information.
For Baltimore, it marked the second-largest blown fourth-quarter lead in John Harbaugh's 16 years as coach.
"I think I can speak for everybody, when [we jumped out to a 14-0 lead] nobody thought that [the Browns] were going to win the game, and that's what happened," Ravens defensive tackle Justin Madubuike said. "So it was definitely just shocking, frustrating."
"Kept on trying to tell y'all once he hits his stride, he's gonna be back to his previous ways. We're just seeing a glimpse into what he can be and who he is." Browns DE Myles Garrett on Deshaun Watson
The Browns lost their first meeting with the Ravens 28-3 in Cleveland. Watson did not play in that Week 4 matchup as he was recovering from a rotator cuff strain that kept him out of nearly four games (he played one quarter in one of them before exiting). He finally returned last weekend in Cleveland's 27-0 win over the Arizona Cardinals.
Watson has largely struggled since being traded to the Browns last year. He entered Sunday's game ranked 23rd in the league in QBR (44.5).
The Browns were without injured starting offensive tackles Jedrick Wills Jr. and Dawand Jones against the Ravens. But even with a depleted line, Watson kept the offense moving in the second half against a Baltimore defense that came into the game ranked No. 1 in efficiency.
"Kept on trying to tell y'all once he hits his stride, he's going to be back to his previous ways," Browns All-Pro defensive end Myles Garrett said of Watson, who led the NFL in passing in 2020 while with the Houston Texans. "We're just seeing a glimpse into what he can be and who he is."
Bitonio acknowledged the victory was a "big statement" for the Browns, who face the 6-3 Pittsburgh Steelers at home in another key divisional matchup next weekend.
Garrett called Sunday's win a "defining" one.
The Browns have momentum heading into the second half of the season, Watson added.
"We got to continue to just channel all this energy," he said, "this power we have right now, and keep pushing forward."
ESPN's Jamison Hensley and ESPN Stats & Information contributed to this report.