EXCLUSIVE

YG's facing a felony robbery charge for allegedly yanking a dude's chain off his neck and taking off with the diamond-laced pendant ... and we're told there's video evidence too.

The rapper was in a Vegas court Thursday after a warrant was issued for his arrest and he turned himself in. YG was hit with felony robbery and he was released after posting $20k bail.

Vegas police tell us ... the incident went down in late May at the Cosmopolitan around 4 AM, after a man approached YG and asked for a pic but was turned away by his crew. We're told the guy told YG he's not a real celeb ... and casino's surveillance footage shows YG retaliating by yanking a chain off his neck.

We're told the chain fell to the ground, and YG is then seen on video picking up the necklace pendant -- valued somewhere between $3,000 and $9,000 -- before taking off.