Enzo Maccarinelli floored Marcelo Dominguez in round nine to claim the interim WBO cruiserweight title.
Maccarinelli sends Dominguez to the floor with a perfect uppercut
Injured champion Johnny Nelson must now fight Maccarinelli before September or risk being stripped of his crown.
"I went out there to box him and be patient and then I stepped up to finish him off," said Maccarinelli.
And on a good night for Welsh boxing, Gary Lockett retained his WBU middleweight title by unanimous decision over England's Ryan Rhodes.
Maccarinelli became the first man to stop the 36-year-old Dominguez when he sent him dramatically toppling backwards into the ropes in Cardiff.
Dominguez, from Argentina, had shrugged off plenty of big punches from Maccarinelli before the Welshman finally unloaded the knockout blow.
All true champions get up and win and if we have a re-match I will be 50% better next time
Promoter Frank Warren said: "Johnny Nelson is the champion and we will be looking to put that fight on in September. If Johnny is not ready he will be stripped according to WBO rules."
Lockett, from Cwmbran, was put down for the first time in his career by a Rhodes right hand in the 10th round of their bout.
But he clambered up on unsteady legs before recovering well to shade the remainder of the fight and snatch the verdict.
Rhodes had been dumped onto one knee in the opening round by a flash Lockett left, and that helped the favourite eventually win a 115-112, 114-112, 117-111 decision.
Lockett said: "My knockdown in the first round did not help because I just loaded up looking for the finish and my game-plan went out of the window.
"All credit to Ryan because when he knocked me down I have never felt like that in my life. But all true champions get up and win and if we have a re-match I will be 50% better."