JUSTIN BIEBER SUED

The man claiming Justin Bieber's thugs beat him to a bloody mess at a Southampton nightclub is finally taking him to court and demanding the singer pay up.

As TMZ previously reported ... Biebs' crew was investigated by police for the violent brawl last year at South Pointe nightclub that allegedly left 22-year-old Wayne Rennalls covered in cuts and bruises.

Cops weren't interested in Bieber -- since he didn't touch anyone -- but in a new lawsuit Rennalls says Justin's security team was on his payroll, so it's on him to make it right financially. In the docs, he also puts the club on blast for allegedly slipping drinks to then 19-year-old Bieber.

Rennalls says his pain and stress are worth big bucks. He's suing for an unspecified amount of cash from JB and the club.