Marlon Wayans Watched Nick Diaz Beat a Dude's Ass In the Street

And now ... "Street Fighting Stories with Marlon Wayans!"

This time, Marlon is telling us all about the time someone picked a fight with UFC star Nick Diaz -- and it ended with VIOLENCE!

Marlon was leaving Catch in West Hollywood over the weekend -- where Nick Diaz had also been hanging out -- and it reminded Marlon about the one dumbass who tried to fight the Diaz Brothers in front of him.

"I said 'Brah, you f***ing with the wrong dude.' He goes 'F*** that bro. You don't know me. I'm gonna f*** [him up].'"

Marlon says he TRIED to warn the wannabe tough guy ... to no avail.

"Nick grabbed his ass ... Pop! Pop! Pop! Three elbows. Pop! Pop! Two knees. Fight was over. Blood everywhere."