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.

Rihanna missed the action — she was already inside her car when the knockout punch was thrown.