Justin Bieber -- Rear-Ended in West Hollywood

Justin Bieber just got rear-ended ... and it looks like a case of quien es mas macho.

Bieber was driving his red Ferrari in West Hollywood early Tuesday afternoon when he noticed a photog following him in a Prius.

We're told an annoyed Bieber slammed on his brakes and the photog crashed into him.

Bieber wasn't hurt, but he's pissed, tweeting, "We should have learned from the death of Princess Diana."

As for the cars ... the Ferrari was scratched ... nothing major. And let's face it ... who cares about the Prius.