Dillon Danis appeared to urinate on an Arsenal shirt in an apparent dig at KSI.
Danis, 30, is set to take on Logan Paul at the AO Arena in Manchester next weekend, with the bout on the undercard of KSI's showdown against Tommy Fury.
Much of the fight's unsightly build-up has seen Danis launch personal attacks on his fierce rival Paul, though he has also taken the time to aim a bizarre jibe at KSI.
KSI is a passionate Arsenal supporter and MMA fighter Danis looked to wind him up by filming a video of himself seemingly urinating on the club's red home jersey.
In the clip posted on X, formerly Twitter, Danis held the shirt aloft toward the camera before placing it on the ground, turning around and pulling down his trousers.
'Hey KSI, I heard you like Arsenal so I got you a little gift,' he said.
The camera then zoomed in slightly on the sodden jersey before the video ended.
The lead-up to Danis' match-up against Paul has been deeply controversial.
He was hit with a restraining order over harassment after posting old pictures and videos of Paul's fiancee, Nina Agdal, and made a series of X-rated comments.
Model Agdal alleged she had 'suffered humiliation, emotional distress and reputational harm' from Danis' posts and has officially filed a lawsuit.
Danis has hinted he may withdraw from the fight, saying: 'Logan bragging about lawsuits. He isn't built for the fight game, this p**** doesn't deserve me. I'm out.'
Mike Perry has indicated he will step in instead. He said: 'It's official guys, I'm in.'
MUTINDO
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Dillon is officially our enemy.. And he may not go far in his career... Coz he doesn't respect people's feelings.. How dare..!!! Mtsiiii... 🔥🔥Gunners For Life... ♥️🤍🔥🔥...