Ex-UFC Champ BJ Penn -- ARRESTED IN HAWAII ... Parking Lot Brawl

Former UFC champ BJ Penn was arrested in Hawaii this weekend ... after reportedly beating a man so bad in a bar parking lot, the guy was hospitalized for a broken eye socket.

Penn had been boozing at a bar in Maui with a friend when he allegedly began to get aggressive with other bar patrons ... and Penn's buddy tried to step in between and calm the fighter down.

But according to HawaiiNewsNow.com, Penn turned on his pal -- punching him in the face and kneeing him in the forehead.

Penn's friend was so badly injured, he was reportedly taken to a nearby hospital where he was treated for a fractured left eye socket.

According to officials, Penn left the scene ... but was taken into custody at a nearby hotel. He was arrested for 2nd degree assault.

Story developing ...