Tyson Fury stopped Nicolai Firtha in the fifth round of their heavyweight contest at the King's Hall, Belfast.
The British and Commonwealth champion dominated the opening two rounds but Firtha had his opponent in trouble with a big punch in the third.
Manchester-born Fury, 23, regained the upper hand in the fourth and referee John Keane halted the fight after two minutes 19 seconds of the next.
For Fury, it was the 16th victory of his unbeaten professional career.
Eleven of those 16 wins have come by knockout.
Firtha, who took the fight at two weeks' notice, survived an early onslaught from Fury but recovered to land a fine right hand, which rocked his 6ft 9in opponent.
A huge right in the fifth had the American in trouble again and this time the referee stepped in.
"He caught me with a good punch and I had to come back from it," admitted Fury after the fight, which had been scheduled for 12 rounds.
"It's all part of being a heavyweight boxer.
"It was an action-packed five rounds for the Irish fans and credit to Nicolai, he's a tough guy, but in the end he was taking too many.
"I'm still learning and I'm just looking forward to my next fight," he added.