It is believed the teenager and three friends were on the bridge from Lanyon Place to East Bridge Street when they were approached by four youths.
A struggle ensued and the teen jumped to escape. He fell 20 feet onto the road below, suffering head injuries.
Four 15-year-olds who were questioned by police have since been released.
The incident on the bridge near Central Station happened at about 2130 GMT on Saturday.
Police appealed for anyone who witnessed the incident to contact detectives at Musgrave Street station on 0845 600 8000 or Crimestoppers on 0800 555 111.
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