West Coast Eagles footballer Murray Newman has been found guilty of fracturing a man's jaw at a Perth nightclub for sleeping with his girlfriend.

Newman has been on trial in the Perth District Court for three days, accused of causing grievous bodily harm to Brett Marris, then aged 20, in November last year.

West Coast Eagles footballer Murray Newman has been found guilty of grievous bodily harm.

Prosecutors alleged Newman had a grudge against Mr Marris because he had slept with Newman's girlfriend when the couple were on a break.

After weeks of threats from Newman on Facebook, the pair saw each other at the Library Nightclub in Northbridge, where they fought for about 10 seconds before bouncers broke up the fight, the court heard.