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.

We broke the story ... the alleged victim is also suing YG, claiming he had his goons beat him up before he snatched his jewelry. The guy wants at least $250k.