The 32-year-old Rees came out swinging at the start but failed to slow Broner, who justified his pre-fight trash talk by twice sending Rees to the canvas before the Welshman’s corner called a halt to proceedings.
Afterwards, Rees had nothing but praise for Broner.
“He’s the best I’ve been in with. He’s world champion for a reason,” he said.
“He was catching me with some great shots and the gameplan went out of the window. When you’re stunned you don’t think properly.
“I’m a warrior, I’m always going to get back up. I’d have carried on until he knocked me out cold.”
Rees did all the work in round one, but Broner — whose shots were less regular but far more powerful — was clearly unperturbed and demonstrated it with some trademark showboating in the second.
The end of the third saw a tiring Rees endure a sustained barrage before Broner put him down in the fourth with a huge right uppercut.
Rees, to his credit, rallied but with the shots raining in during the fifth, his trainer Gary Lockett waved the towel to signal the end.
Broner, who said he would look to face Scotland’s Ricky Burns next, said: “He’s tough as a steak that’s been cooked too long. He’s real tough.”