Russian Sergey Kovalev dominated 49-year-old Bernard Hopkins from the opening bell to win their light-heavyweight unification match.
The unanimous decision gave the 31-year-old the IBF and WBA belts to go with his own WBO title.
Kovalev, who knocked down Hopkins in the first round, had winning scores of 120-107 from two judges and 120-106 from the third.
The defeat in Atlantic City could signal the end of Hopkins's career.
It was the first time that Kovalev had been past the eighth round in his professional career but he shone in the 12th, sending the American against the ropes and landing 38 punches, the most ever against Hopkins in 41 fights analysed by CompuBox.
"I wanted to show fans that I know how to box and I did," Kovalev said. "I tried to go for the knockout in the 12th round. He has great defence. He's the best boxer in my division."
But the Russian now feels the time is right for Hopkins to hang up his gloves.
Born: 15 January 1965, Philadelphia, USA
Turned pro: 11 October 1988 (lost on points)
Pro record: 66 fights, 55 wins (32 KOs), 7 losses, 2 draws, 2 no contests
World titles: Middleweight, 1995-2005 (20 defences); light-heavyweight, 2006-2008, 2011-2012, 2013-2014
"He was a really tough opponent," he added. "He is very good at keeping distance. I really respect him for the fight but he needs to stop his career, I think, because he's already done a lot in the boxing world and he needs to give an opportunity to younger fighters to be champions. I'm next."
However, boxing legend Oscar De La Hoya, 41, who was the last man to be knocked out by Hopkins in September 2004, insists that the American will fight again.
Hopkins is a minority partner with De La Hoya in Golden Boy Promotions, and the world champion at six different weights believes Hopkins has options in a lower weight class.
"I give him a lot of respect," said Hopkins of his opponent after the fight. "We both would fight anyone and that's how we ended up here tonight. That's what brought us together. That's what the people want to see, one title, one belt, one champion.
"When he got hit with some of my shots he would step back. But he used his reach and his distance and that was the key to his victory. He has very good mechanics and patience. It was smart that he stayed patient. He had a really good game plan. I will give him that."