Law enforcement sources in Cleveland tell us Bieber is now the suspect in an incident back in June ... an incident captured on video showing Bieber fighting a guy after a Cavaliers playoff game.

The alleged victim told cops Justin grabbed his sunglasses. The guy then took a pic of Justin with the shades, which he says enraged the singer and sent him into attack mode. He claims Bieber's crew then got in on the action.

The alleged victim says he went to the hospital with various injuries, including a concussion.

As for why it took the victim so long to file ... people connected with him say he was trying to settle up with Justin but he just wouldn't play ball.

For what it's worth ... the alleged victim looks twice as big as Biebs.

Law enforcement sources say the investigation is ongoing and no charges have been filed.