Sean Kingston Sues Penthouse Nightclub ... You Were In on the Assault

Sean Kingston is pointing the finger squarely at Penthouse Nightclub for allowing its security to beat his butt and jack his pricey chains ... so claims the singer in a new lawsuit.

Kingston says security at the WeHo club was involved in orchestrating the assault in which someone yanked the chains off his neck ... totaling $300k.

Kingston says he was in the VIP section when security told him another VIP guest wanted to talk to him. When he approached the other guest, he says he was assaulted, punched and beaned with a champagne bottle.

As TMZ reported ... cops were called to the venue but everyone involved had already skedaddled.

Kingston is suing Penthouse Nightclub for $900k minimum.