Justin Bieber -- New Legal Trouble In Atlanta ... Bodyguard Arrested for Theft

Justin Bieber has just found trouble in Atlanta ... one of his security guards is under arrest for stealing a photog's camera at a fun house ... TMZ has learned.

Justin went to an amusement park Tuesday afternoon, and while he was inside, law enforcement tells us a member of his security team went outside to his Escalade and told waiting paparazzi to stay cool because Justin was about to come out.

We're told the security guy got into it with one of the photogs, grabbed the guy's camera and handed it to the driver. The security guy then allegedly jumped in the car and told the driver to take off.

Cops found the Escalade at the home Justin is renting from rap mogul Dallas Austin and they searched it for the camera, which they located inside. We're told the photog followed the vehicle to Justin's house. Police then returned the camera to him.

We're told the guard is under arrest for theft and cops are deciding whether to arrest the driver.

Law enforcement sources say cops removed 2 backpacks from the Escalade, but one law enforcement source tells us they didn't find any contraband.

UPDATE 3:20 PM PT -- We're told the Escalade has just been TOWED.