The man behind the suit is Christopher Mostello -- who was working security at the T-Mobile Arena on Oct. 6, 2018, for UFC 229 when he got called over by a supervisor to respond to an incident involving Pierce.

Mostello claims he rushed over and was told Pierce had gotten aggressive and physical with the security supervisor -- a woman -- because she had asked for his ticket before allowing him back to his seat.

Mostello claims he was told Pierce grabbed the woman by the arms and pushed her to the side -- so he stepped in front of Paul to try and stop him.

But, Mostello claims Paul didn't stop -- instead Paul screamed "F*ck you" and accused Mostello of being a racist.

Footage of the altercation was captured by other fans in the stands and posted on TMZ Sports -- where you can clearly hear Pierce call the man a racist.

Mostello claims he's NOT a racist -- and he only engaged Pierce in the capacity of doing his job.

Now, Mostello claims his reputation has taken a serious hit and he's been subjected to public embarrassment, harassment and ridicule.

In the suit, filed by attorney Jesse Sbaih, Mostello accuses Pierce of defamation and intentional infliction of emotional distress. He's demanding more than $15,000 in damages plus attorney's fees and other costs.