We broke the story ... a security guard at the T-Mobile Arena in Vegas sued Pierce for defamation and other things over an altercation they got into during UFC 229 in Oct. 2018.

Long story short ... Pierce called security guard Christopher Mostello "racist" -- claiming Mostello (who is white) gave him a hard time while trying to get to his seat.

Pierce's "racist" allegation was captured on video and posted by TMZ Sports.

In his suit, Mostello claimed it was Paul who was acting inappropriately that night -- saying Pierce had gotten aggressive and physical with a female security supervisor -- and Mostello was only doing his job.

Mostello claimed his reputation took a serious hit because of the altercation and Pierce's accusations -- and he's been subjected to public embarrassment, harassment and ridicule.

The case went in front of a judge who ordered Pierce to either respond to the lawsuit or face a default judgment ... and since the court never heard from Pierce, the judge ordered him to pay up.

Pierce isn't out of moves yet ... he can ask the court to set aside the judgment but that's usually reserved for someone who didn't have the means to fight in the case in the first place.

Bottom line, it's an uphill battle for Pierce and it would probably cost him more than $100k to go that route anyway.