Play video content EXCLUSIVE 9/3/18 TMZSports.com

Ex-UFC fighter Melvin Guillard -- who fought the likes of Nate Diaz and Justin Gaethje -- brutally attacked and KO'd a man in a Colorado bar ... and now cops are on the hunt.

TMZ Sports has obtained surveillance footage which shows the 35-year-old getting into it with several people at Bar Standard in Denver around 2 AM on Monday morning.

After some pushing and shoving, you can see Guillard -- with his trademark bleach blonde hair -- cold-cock a bar patron in the face ... knocking the guy out cold before he hit the floor.

The victim was clearly unconscious and people ran to tend to him -- while a female patron tried to attack Guillard in retaliation. Guillard hit her too.

TMZ Sports spoke with a Denver Police Dept. spokesperson who told us several witnesses were able to identify the attacker as Guillard ... and now a warrant has been issued for his arrest.

Guillard was a pretty promising MMA fighter when he first started out -- appearing on "The Ultimate Fighter" season 2. He famously scored a victory against UFC stud Jeremy Stephens back in 2010.

But, he also got his ass kicked a lot ... chalking up Ls to guys like Nate Diaz, Israel Adesanya, Justin Gaethje, Michael Johnson and Donald Cerrone.