Rampage Jackson Grants Fan Request -- Yes, I'll SLAP You In the FACE!

UFC face pummeler Rampage Jackson got one crazy request from a ballsy fan last night -- dude wanted to get SLAPPED IN THE FACE ... and Rampage obliged.

Rampage was leaving Greystone Manor nightclub in L.A. ... when a fan with a pain fetish approached Jackson and asked to be cracked in the mouth.

Never one to deny a fan ... Jackson bestowed the 5-finger honor across the fan's jaw -- and the fan couldn't have been HAPPIER!