Chris Brown -- Rear-Ender ... He Hardly Knew Her!

Chris Brown has yet another legal problem to deal with, after crashing his car in Los Angeles near Toluca Lake.

The accident happened an hour or so ago ... The photog tells us Chris was driving his Range Rover with his GF du jour Karrueche Tran, when he rear-ended a Mercedes.

Chris parked in front of the Mercedes and then exchanged info with the Mercedes chick. The photog says damage to her car was "medium" ... as opposed to rare.