Justin Bieber Involved In West Hollywood Car Crash

Justin Bieber was smacked from behind in his G-Class Friday night ... and the L.A. County Sheriffs Department got involved.

Bieber was on Sunset Boulevard just before 9 PM in West Hollywood when he was rear-ended by a Range Rover. Law enforcement sources tell us an OnStar system from one of the vehicles alerted them to the crash.

We're told officers stood by while Bieber and the other driver exchanged info ... but cops didn't take a report.

Bieber appeared to be alone, but he's recently ditched Selena Gomez and hanging out with Patrick Schwarzenegger's girlfriend, Abby Champion's sister, Baskin.