Former WBA light-welter champion Rees' quest to become a two-weight world title-holder saw him overpowered and outgunned by the fast-rising Broner, who weathered a good start from the challenger and picked him off with powerful strikes.
An inch-perfect right uppercut felled Rees in four and a controversial second knockdown came in the fifth when 'The Problem' caught 'The Rock' with a stiff body shot when it appeared the referee Earl Brown may have been ordering a break.
There was no confusion over the finish, however, as the American rained down more abuse on the Brit until trainer Gary Lockett offered the towel.
Though Rees' record fell to 37-2-1, he gained a following in Boardwalk Hall with his fearless approach to the fight against world-class opposition and earned a strong endorsement from Broner, a rare moment of respectful humility from the brash Ohio resident after the bout.
Rees began in typical Rees fashion, pushing right up to his heavily-fancied adversary and even out-landing him to take the opening round.
Though the Welshman was able to keep the approach going to an extent in the second, Broner began to settle into his opponent's rhythm and edged the round on the Yahoo!-Eurosport scorecard with some excellent shots.
The margin of class began to show in the third as Broner's showmanship and stinging counter-strikes overcame the crowd's slow-burning support of the fearless underdog, who failed to cover up at the right times.
The breakthrough came in the next round as the American's shots began to hurt Rees and after unwisely going unside straight after taking some choice lands the challenger was felled by a cracking uppercut,
Rees had no trouble answering the count but was fortunate not to hit the canvas again before the bell - though he would be sent down again in the fifth.
After rediscovering some of the decent head shots he had put upon Broner in the early stages, Rees slumped from a heavy body shot with the referee's subsequent count confusing those in attendance, who could have sworn they were watching Brown call for a break moments earlier.
Nonetheless, the British boxer duly answered the count once more but his girlfriend at ringside saw the writing on the wall and made her way backstage. So did trainer Lockett, who hesitated several times before finally withdrawing his man as Broner launched another brutal offensive.
Broner is expected to enter the world pound-for-pound top 10 before the year is out after showing yet again why he is tipped to be boxing's next massive star.
Rees however, while disappointed with the loss and a few key mistakes he had made, had earned the respect on a global stage that he had deserved for much of his 15-year career.
"I made a lot of mistakes. My skillset is better than that so I'm disappointed," Rees said.
"In the third and fourth his power and speed caught up to me and that was the end of it.
"I'm a warrior, I'm always going to get back up and I would have carried on until he knocked me out had Locky not put the towel in."
Broner played up to his persona - which pre-fight saw him rap his way to the ring with his 40-strong entourage - after the victory with his usual hair-brushing and name-forgetting antics, but paid tribute to one of the biggest obstacles of his 26-0 career so far.
"Rees is tough as a steak that's been cooked too long," the champion said. "Have you tried eating a steak like that?
"Two world class fighters fought and we put on a great show. I picked the better shots and he was a little careless and that was the difference. I'm not surprised he kept getting up because he's that tough."
On the undercard, Sakio Bika earned a shot at WBC super-middleweight champion Andre Ward with a unanimous decision win over previously-unbeaten Nikola Sjekloca in a final eliminator with scorecards of 120-108, 119-109 and 118-112.
And Demetrius Hopkins - nephew of legendary boxer Bernard, won the United States junior-middleweight strap from Charles Whittaker when the 39-year-old retired in his corner after six one-sided rounds.