Tommy Fury stopped Przemyslaw Binienda in just 62 seconds in his first bout since appearing on Love Island.
The 20-year-old - the younger brother of former world heavyweight champion Tyson - had not competed since March.
And against Binienda, who entered with 26 defeats from 28 bouts - a solid right hand to the temple floored the Polish fighter inside 20 seconds.
An onslaught delivered a second knockdown before the bout was waved off with Binienda being overwhelmed.
Light-heavyweight Fury was trained by his father John for the third bout of his professional career after parting company with former two-weight world champion Ricky Hatton.
He was cheered on from ringside by partner Molly-Mae Hague - who he met on ITV 2's Love Island show - and told BT Sport after his win: "It feels absolutely amazing.
"I wasn't looking for a knockout at all, I was going to box to a gameplan. I would not be here without this man to my left, my dad. It's father and son and we are going to the top.
"This year I have gained a lovely soul mate in Molly and I've ended the year on a big knockout. More of the same in 2020."