A source close to the UFC superstar tells TMZ Sports ... as Izzy was parking his car on a roadway in his home country, the guy came out of nowhere and started calling him the N-word.

Soon after, our source says the man challenged Adesanya to a fight.

In video filmed by a witness -- which has now gone viral on social media -- it's hard to make out what either dude is saying ... although it's clear both parties were pissed at one another.

In fact, it appears Adesanya spits on the man at one point.

Fortunately, the altercation never turned physical ... and the two men eventually went their separate ways.

Adesanya is expected to address the incident at an anti-street violence charity later Thursday.