The victim suffered cuts and bruises to his head after being set upon by the gang, thought to be aged between 14 and 16.
He had been with two friends, who suffered minor injuries, when he walked toward a toilet block in Havant Park and was spoken to by youths sitting on a bench.
They attacked as he walked to the toilets. Police said up to 30 people were in the area at the time.
Officers arrived within 90 seconds of the attack at 9.30pm but the youths had dispersed.
Emergency crews battling to save someone’s life on Southsea seafront
Thunderstorms in Portsmouth yellow weather warning: The Met Office forecast for the next three days in city, Fareham, Gosport, Havant and Waterlooville and Hampshire
Police update on probe into death of ‘Wiggy’ Symes after fatal dog attack in Fareham
Southsea seafront incident: Police update after man found 'unresponsive'
River Ems crisis: Westbourne villagers plead with Portsmouth Water to introduce a hosepipe ban
The victim is recovering at home but police want to hear from anyone who may have mobile phone footage of what happened.
Police said the youths were wearing shorts and t-shirts, and some were not wearing tops.
PC Peter Vincent, from Havant’s Neighbourhood Policing Team, said: 'We believe that there a number of people who were in Havant Park at the time who may have seen what happened. We arrived at the scene within 90 seconds of the initial report being made to us, but the group that was in the area had already dispersed.
'I’m keen to locate any witnesses who have not spoken to us already.
'There is also a possibility that parents may have seen teenagers arriving home with minor injuries last night, could this be connected to our investigation? Please call us on 101 if you think you can help.'
Police have previously launched Operation Syren combat anti-social behaviour in the town centre.
Anyone with information should call police on 101 quoting 44170262173.