Greg Paul was outside the event when the incident occurred, after both his sons Logan Paul and Jake appeared at London’s York Hall ahead of his fight with KSI next month.
Video footage shows the moment a large crowd pushed against security.
One man managed to break through and chase after the 54-year-old, before striking him in the head.
Greg can then be seen fleeing the scene, leaving his hat behind.
Reports also emerged he was spat on.
It comes after Logan stormed out of the press conference after a barrage of abuse from KSI.
The footage was uploaded to YouTube channel Izadi, where it has racked up more than 200,000 hits.
And fans were quick to share their opinions.
“He could have got brain damage from that,” one claimed.
Another added: “That’s what you get for talking smack.”
And a third revealed: “I hate the Paul family but that is too far."
The pair, who are both YouTube stars, are due to step inside the boxing ring on August 25.
It will be KSI's second time in the ring against a YouTube star, after beating Joe Weller in February
Daily Star Online has reached out to the Paul family for comment.
