Floyd Mayweather threw punches and left Jake Paul with a black eye as they brawled in Miami
Mayweather was promoting his fight with Jake's older brother Logan when the younger sibling confronted him.
He demanded his own fight with the ring legend and then stole Mayweather's cap.
Mayweather ran after the social media star and could be seen throwing several punches and at one stage Paul was seen picking himself up off the ground.
He was later heard shouting, "I'll kill that motherf*****. When I catch you I'm going to kill you" as he was led away through the stadium while Paul tweeted simply, "Gotcha hat".
The fracas followed a face-off between Mayweather and Paul ahead of their exhibition bout at the Hard Rock Stadium on June 6.
During the event, Mayweather had vowed to fight both Paul brothers on the same night.
Jake Paul has three professional fights on his record, against fellow social media star AnEsonGib, former NBA star Nate Robinson and former UFC wrestler Ben Askren.
His brother Logan, who will take on Mayweather over six rounds, has fought once in the paid ranks, in his rematch with felllow YouTuber KSI.
When they faced off, Paul said: "I think you've bitten off more than you can chew. Why come out of retirement just to get re-retired."
Mayweather added: "We're going to come out, entertain and have fun.
"It's a fight for him but for me it's all about fun.
"The last time I fought in an exhibition in Japan it was only two minutes, hopefully this time it will be a bit longer.
"I've been here so many times and I just know what it takes."
Mayweather last fought in 2018 when he took on Japanese kickboxer Tenshin Nasakawa, knocking him out in the first round in Tokyo.
He previously took his unbeaten record to 50-0 with a 10th round stoppage of UFC star Conor McGregor.