UFC legend Don Frye got into a physical altercation with a fan at UFC 270, punching the man in the face ... this after the retired MMA star and pro wrestler tells us the guy bumped into him and challenged him to a fight.
TMZ Sports obtained video taken at the Honda Center in Anaheim, CA Saturday night ... showing 56-year-old Frye jawing with two men as someone who appears to be security attempts to keep the peace.
Frye is seen popping the guy in the head with a jab (clearly not at full force) ... sending the man's head snapping back.
We talked to Don ... who tells us the incident started when the guy, who he believes was drunk, bumped into his hat. The men then had a spirited conversation, which Frye says culminated with the man challenging him to a fight.
You can only assume the man didn't know he was challenging Frye ... one of the UFC's earlier stars, and most powerful strikers.
Frye won UFC 8 and Ultimate Ultimate '96 tourneys ... and was the runner-up at UFC 10. Over his incredible career, Don beat guys like Ken Shamrock, Tank Abbott and Mark Hall.
Frye -- who recently went on Joe Rogan's podcast and explained how he got into MMA -- was inducted into the UFC Hall of Fame in 2016.
In addition to the UFC, Don fought in Pride and K-1. He also boxed and kickboxed professionally, and was one of Japan's biggest wrestlers during his time with New Japan Pro-Wrestling.
Don was not arrested -- and stayed to watch the remaining fights -- including heavyweight champ Francis Ngannou defending his title.
As for the fan who was popped with the jab ... it's unclear if the man was removed from the arena.