Conor McGregor appeared to spit on a Khabib Nurmagomedov supporter in Las Vegas recently ... with video of the heated altercation quickly making rounds on social media.

The clip shows Conor and his crew walking by Caffè Al Teatro inside the Wynn ... when someone nearby yelled out his support for Notorious' arch nemesis.

Conor did a heel turn and walked right up to the man responsible for the "Let's go Khabib" remark ... and seemingly let some saliva fly in his direction.

It's unclear if he made contact with the heckler, but Conor is heard saying, "I spit in your face, what you do?? Nothing."

The interaction could land Conor in some hot water -- spitting on someone is considered a battery in Vegas ... and since there's video, the guy could easily go to the cops.

It's unclear if anything else happened after the video cuts out ... but Conor is shown chatting with his crew as they all walk away.

Conor's been all over the place lately -- after attending a BKFC event in Philly, he sized up Brook Lopez at the Milwaukee Bucks game ... and hit up the BYU basketball game as well.

On top of that, he also raised eyebrows with a fiery social media post ... in which he used the n word while going after Khabib on X.