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.
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.
But Newman claimed he acted in self-defence against a taller man who grabbed his throat.
It took a jury about three hours to convict Newman on Wednesday.
During the trial, the threatening Facebook messages were shown to the jury including one where Newman told Mr Marris to "start digging a grave".
Newman also called Mr Marris a "little girl" and asked why he did not want to meet "for a crack".
In another message, Newman wrote: "I won't rest til you ey (sic) hurt."
Judge John Staude ordered pre-sentence and psychological reports.
He also scolded Newman as he told the footballer he had been found guilty of a serious offence that could carry a jail term.
"Take your hands out of your pockets," Judge Staude said.
But he noted Newman had no prior record and was young when he committed the offence.
Newman will next appear in court on March 7 and has had his bail renewed, with conditions including not entering licensed premises unless he's on official West Coast Eagles business.
Newman has been supported in court for the past three days by West Coast Eagles teammates including captain Darren Glass, Dean Cox, Nic Naitanui, Josh Kennedy and Sharrod Wellingham.