Rihanna -- Bodyguard Throws Knockout Punch to Pap's Face

First Drake vs. Chris Brown ... and now ANOTHER fist fight over Rihanna ... but this one ended quickly.

It all went down Sunday night in London ... when a paparazzo (not a TMZ guy) got too close to Rihanna ... and one of the giants protecting the pop star unleashed a right cross that connected ... HARD.

Keep your eyes on the huge guy with the bald head ... who was trying to clear out a space in front of Rihanna's ride as she tried to exit the Whiskey Mist nightclub.

The bodyguard wasn't in the mood to talk things out with the photog ... and fired off one punch that sent the shutterbug down for the count. It's unclear if the pap ever got up.