Joe Fournier Officially Appealing KSI Loss After Controversial KO

Joe Fournier is officially appealing his loss to KSI ... just two days after the YouTube star scored an (albeit controversial) knockout victory over the English businessman/boxer, the Professional Boxing Association announced Monday.

The PBA confirmed it received a request from Fournier to review/appeal Saturday night's decision at London's Wembley Arena ... a fight he lost by second-round knockout. However, Fournier says he was put down by an illegal strike, an elbow, not a punch ... and therefore he shouldn't have lost the fight.

Now, representatives for Fournier and KSI will have an opportunity to go before the PBA and state their cases as to why the result should or should not be changed after the fact.

Moments before KSI's elbow came into play, he landed a looping right on Fournier's ear, a punch that appeared to wobble JF. Then, just seconds later, 29-year-old KSI sent Joe crashing to the floor.

Question is, was the strike legal? Fournier believes no.

After the fight, KSI tweeted that the outcome was inevitable, but he did acknowledge there was "unintentional contact made with his forearm" during the contest.