Sean Kingston: Dudes Jacked My $300k Chain!!

Sean Kingston's got an accessories problem ... someone straight up yanked a $300k chain off his neck ... so claims the singer.

Two suspects approached Sean inside Penthouse Nightclub & Dayclub in L.A. and somehow the altercation turned physical ... they began punching him before one forcibly took the pricey chain -- this according to the police report.

By the time cops arrived Sean and the 2 suspects had split. We're told someone from Sean's camp filed a police report, and an investigation will be launched.