Lawrence Okolie stopped Lukasz Rozanski in the first round to claim the WBC bridgerweight title.
The Briton floored the Polish fighter three times in his homeland before the referee ended the bout.
A powerful right hand from Okolie put Rozanski on the canvas for the first time and he was quickly down again after being caught.
Okolie then pounced with a flurry of shots to knock Rozanski down for a third and final time.
“This was the best moment in boxing for me so I’m super happy,” Okolie told Sky Sports.
“New weight, putting the KO back in Okolie.”
Okolie was WBO world champion at cruiserweight before losing to Chris Billam-Smith last year.
While his fight with Rozanski was billed as a world title bout, bridgerweight is a new division, boxing’s 18th overall, and is not recognised by most of the boxing world.
The sanctioning body WBC created it in November 2020 to bridge the gap between cruiserweight and heavyweight.
Bridgerweight begins at 14st 4lb (90.7kg), with an upper limit of 16st (101.6kg).