Logan Paul Guarantees Knockout in KSI Rematch

Logan Paul says he has unfinished business with KSI ... telling TMZ Sports he's still gunning for a rematch with the fellow YouTuber -- and he wants a KNOCKOUT.

We spoke with the social media superstar out in Beverly Hills over the weekend about his plans following the massively successful first fight earlier this year ... and he's fully expecting the rematch to happen.

That is ... "if KSI doesn't bitch out," Logan says.

Paul says he's already training for the rematch ... and wants it to go down in either L.A. at Staples Center or in NYC at Madison Square Garden.

Neither Logan or KSI could come away with the win in August ... but Paul guarantees that won't happen again.

"Next fight, absolutely gonna knock him out. He's not gonna make it past 2 rounds."