KSI secured bragging rights over YouTube rival Logan Paul with an enthralling victory in their Los Angeles rematch.
The 26-year-old won by split decision in an eagerly-anticipated rematch that lived up to the billing at Staples Center.
The pair fought to a draw in their first fight in Manchester last August, and KSI got the better of his rival in this rematch.
KSI won 57-54 and 56-55 on two card while Paul won 56-55 on the other card.
Both fighters came out swinging from round one with very little to choose between them for the opening rounds.
KSI thought he had Logan Paul on the floor in the third round, but the referee ruled it as a slip and not a knockdown.
And the Brit was the one on the canvas in the fourth round with a vicious right hand but Paul was then adjudged to have hit his man while he was down.
KSI was on wobbly legs and survived until the bell before finding his feet again in an even fifth round.
The fight went all the way to the sixth and final round with both men desperate to do enough to secure the decision in their favour.
The crowd gave the fighters a standing ovation, but they didn't touch gloves after the final bell following a bad blooded build-up.
Started from round 1 this time. Aggression, persistence & tenacity were the keys & you unlocked the doors. Congratulations @KSIOlajidebt- Chris Eubank Jr (@ChrisEubankJr) November 10, 2019
They eventually embraced after the decision came in favour of KSI at Staples Center.
He said: "I have never worked so hard man. In the ring, in camp, it was tough.
"KSI last time was scared. I was scared to get close to him and fight him. This time I just went in."
Paul, meanwhile, was angry with the decision to dock him two points for hitting KSI when he was down in the fourth round which probably cost him the fight.
He said: "I thought it was a legitimate knockdown. I didn't get a warning and I got two points taken away from me.
"That's the reason I lost tonight. I would love to contest the commission because I think I should have got a warning. I don't think it was fair at all."