BOSTON — The Chicago Blackhawks pulled off perhaps the most improbable comeback in the history of the Stanley Cup finals on Monday night. Trailing the Boston Bruins, 2-1, with 76 seconds left, the Blackhawks erupted for two goals 17 seconds apart to stun the Bruins, 3-2, and win their second Cup in the last four seasons.

“This goal, this ending — nobody saw it coming,” said Blackhawks Coach Joel Quennville, after a finish that strained credulity. So sudden and late was Chicago’s rally that even the Blackhawks themselves seemed not to believe what they had just seen. 

Milan Lucic put the Bruins ahead, 2-1, with 7 minutes 49 seconds left in regulation, seemingly forcing a Game 7 in Chicago on Wednesday.

But then lightning struck. With 1:16 left, Bryan Bickell finished a feed from Jonathan Toews, knocking the puck past Boston goaltender Tuukka Rask and tying the score at 2-2. With 59 seconds to go, Dave Bolland beat the sprawling Rask after a rebound off the goal post, and just like that, the Blackhawks were ahead. 

“How can you call that?” Toews said, beaming, in the moments after victory. “We knew we needed just one bounce there. Obviously, that was a big goal for them to go up 2-1. But you never know what can happen, so you don’t stop playing until the end.”

Moments later, Toews was lifting the Stanley Cup over his head in triumph, the first Blackhawks captain in the club’s 87-year history to win it twice. 

“These are the feelings you live for,” said his teammate, Patrick Kane.

Bolland’s goal was the latest Stanley Cup-winning goal scored in regulation time, breaking the mark set in 1929, when Boston’s Bill Carson scored to win the Cup with 1:58 left in the deciding game against the Rangers. 

The Blackhawks also became the first club to win a Stanley Cup-clinching game in regulation by overcoming a deficit in the final two minutes.

On the Bruins side, shock mingled with dejection.

“It’s a tough way to lose, tough way to lose a game, tough way to lose a series,” said Zdeno Chara, the Bruins’ towering 6-foot-9 defenseman. Chara was especially glum. He had been on the ice for 10 of the Blackhawks’ last 12 goals in the series, although not Bolland’s Cup winner. 

Bolland, a third-line center, was surprisingly blasé about what he had done.

“I could always imagine the season ending this way,” he said, a sentiment that was as credulity-straining as the goal he scored. He was asked the difference between winning the Stanley Cup this season and in 2010.

“What’s the difference?” he said. “Nothing.”

Bolland and Bickell’s goals ended this classic, almost unbearably tense series, in which three games went to overtime and every game was razor close. Kane was voted the winner of the Conn Smythe Trophy as the playoffs’ most valuable player, becoming the third straight American to receive the honor. He had 9 goals and 10 assists in the postseason. 

It was the Blackhawks’ fifth championship, joining their triumphs in 1934, 1938, 1961 and 2010.

Chicago trailed, 1-0, and did not mount a real threat until Toews, motoring at full speed after an undisclosed injury that knocked him out of Game 5 on Saturday, scored at 4:24 of the second period.

In Game 6, players dropped left and right as they put everything on the line with the Cup at stake. Bruins center Patrice Bergeron played a full game despite a series of injuries that did not become known until after the game. 

Blackhawks forward Andrew Shaw played despite taking a hard shot to the face; later he carried the Cup around the ice while bleeding through his stitches. Blackhawks defenseman Niklas Hjalmarsson left for a short time after blocking a shot. Bruins forward Jaromir Jagr was rattled by a check from Bolland and missed most of the second period. Chicago’s Marian Hossa and Boston’s Nathan Horton played on through nagging injuries, as they have throughout this series. 

The Blackhawks became the first team of the N.H.L.’s salary-cap era to win the Stanley Cup twice. After their 2010 victory, General Manager Stan Bowman had to trade away or decline to re-sign nine players from the championship roster because their rising salaries would have pushed the team over the cap.

On Monday, there were eight holdovers from that 2010 team on the ice: Toews, Kane, Bolland, Hjalmarsson, Patrick Sharp, Hossa, Duncan Keith and Brent Seabrook. 

“There’s something about our core,” said Kane, who scored the Cup-winning goal in overtime at Philadelphia in 2010, another Blackhawks championship won on the road in Game 6. “Hopefully we can stay together a long time, because that’s two Cups in four years, and we seem to only be getting better and better as players as time goes on.”

“It’s unbelievable to be in this situation,” he added. 

Quenneville is also a holdover from 2010, and his Blackhawks teams have excelled at postseason crunch time. Their record in Games 5, 6 and 7 of playoff series since he became the coach in 2009 is 19-4.

Chicago rolled to the Stanley Cup. The Blackhawks set an N.H.L. record by going unbeaten in the first 24 games of the 48-game season, half the schedule. They went on to win the Presidents’ Trophy as regular-season champions, and on Monday became the first top regular-season team to win the Stanley Cup since the 2008 Detroit Red Wings. 

They kept rolling in the first playoff round, beating Minnesota in five games. But in the second round, they fell behind the Red Wings, 3 games to 1, and won three straight games, two on the road, to overcome the deficit. Seabrook scored the series winner, in Detroit, in Game 7 in overtime.

In the conference finals, the Blackhawks ousted Los Angeles, the defending Cup champions, in five games. Then came the Bruins, a team they had not faced in a playoff series since 1978, and never in the Cup finals.

“All the hard work pays off, all the way through,” Kane said. “Coming back from 3-1 to beat Detroit, down 2-1 tonight — this is an unbelievable team.” 

It was a series worthy of two big cities with long love affairs with hockey. Bruins flags and sweaters were everywhere in Boston, as Blackhawks flags and sweaters were in Chicago. The Michael Jordan statue in front of United Center wore a Blackhawks jersey. At the Boston Public Garden, the “Make Way for Ducklings” statuettes wore little Bruins sweaters.

The Bruins, champions in 2011, had an improbable ending of their own this postseason, in the first round against Toronto. Trailing by 4-1 with 11 minutes left in Game 7, they rallied to force overtime, scoring twice with their goalie pulled in the last 82 seconds. Bergeron, who tied the game with 51 seconds left, scored again in the extra session to win the series. 

That was an improbable finish that delighted Boston fans, and helped lift the spirits of a city still recovering from the aftermath of the Boston Marathon bombings.

“In the back of our minds, we wanted to do it for those kinds of reasons, the City of Boston, what Newtown has been through, that kind of stuff,” Coach Claude Julien said. “It hit close to home, and the best way we felt we could try and cheer the area was to win a Stanley Cup. I think that’s what’s hard right now for the players. We had more reasons than just ourselves to win a Cup.”