Nick Carter Arrested in Key West

Nick Carter was arrested in Florida on Wednesday night ... TMZ has learned.

According to an eyewitness on the scene, cops took Nick into custody down in Key West at the Hog's Breath Saloon -- a pretty famous bar in the area. We're told Carter was involved in some kind of scuffle inside the bar, and eventually the cops showed up and busted the Backstreet Boy.

He was arrested at 8:13 PM and booked for misdemeanor battery. Interestingly, arrest docs list Nick's occupation as "None." Either he was really humble during his arrest ... or the Key West officers just aren't BSB fans.

4:30 AM PT -- The secret's out ... Nick's arrest report now says, "Singer" for his occupation.