Justin Bieber couldn't leave his cell phone alone while he was operating a motor vehicle ... so now it's gonna cost him.
JB was cruising around Beverly Hills in a Mercedes G-Wagon Saturday when a cop spotted him on his cell and pulled him over. According to law enforcement ... he was issued a citation for using his phone while driving. It's unclear if he was texting or talking.
Cops tell us Justin was calm and cooperative, took his ticket and moved along without further incident.
FYI -- a cell phone ticket is $162 in LA on average ... so the Biebs should have no problem paying the fine.