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Ex-UFC fighter Melvin Guillard knocked out a man on the street in Colorado while working as a bouncer for a popular bar ... and TMZ Sports has now obtained video of the violent punch.

Guillard was working for the Ginn Mill in Denver back on June 15, 2018 ... when a scuffle broke out inside of the bar.

You can see in the video, bouncers wrestle with a man to get him outside ... when suddenly, another male approaches the commotion.

That's when Guillard goes after the guy, pushing him back with one arm ... before socking the dude straight in the jaw.

The punch immediately crumpled the guy ... and he lay lifeless on the sidewalk for nearly 30 seconds before bystanders were able to help him up.

The guy suffered undisclosed injuries in the attack ... and pressed charges -- with cops ultimately arresting 36-year-old Guillard for felony assault.

Guillard fought the charges in court for a year ... but he just cut a deal with prosecutors this summer to have the case closed.

As part of the agreement, Guillard pled guilty to one count of assault in the second degree -- but was only hit with probation -- plus fines and fees -- for the crime.

Guillard had been out of the professional fight game while his case played out in the court system ... but he made his return to MMA action last month in the Bare Knuckle Fight Championship. He lost to Isaac Vallie-Flagg via technical knockout.