The male fell ill on Saturday evening outside The Astoria in Guildhall Walk.
Paramedics were seen attending the scene at 9.15pm wheeling the man away into an ambulance.
He was transported to Queen Alexandra Hospital, in Cosham.
A spokesman for South Central Ambulance Service said: ‘We were called by one of the security team at The Astoria on Guildhall Walk who informed us that an adult male had passed out outside their premises.
‘The ambulance we sent took him to the QA Hospital.’