Police said the 17-year-old was walking along Great Victoria Street near the junction with Ventry Street on Monday evening, when a car containing a number of men pulled up alongside him.
The men then assaulted the victim knocking him to the ground.
It is believed a hammer may have been used to strike him on the back.
The gang left in the vehicle and the teenager was able to raise the alarm.
Police said he was taken to hospital, but his injuries were not thought to be serious. They said they were investigating a motive for the attack.
A teenager has been attacked and knocked to the ground by a gang of men armed with a hammer in Belfast city centre.