Emergency services were called to the closed Odeon cinema in London Road, North End, yesterday evening.
Firefighters attended the scene as well as paramedics from South Central Ambulance Service.
The boy, who is 13, was taken to Southampton General Hospital and is said to have serious but not life-threatening injuries.
A spokeswoman for the police explained: ‘We were called at 6.04pm yesterday evening (Wednesday 15 September) to a report that a 13-year-old boy had fallen from the roof of a disused building on London Road.
‘Officers attended and the boy was taken to Southampton General Hospital with serious injuries. These are not thought to be life threatening at this time.
‘We will work with relevant local partners and the building’s landlord on changes that can be made in order to further secure the location and prevent incidents of this nature occurring in the future.’
A spokesman for Hampshire Fire and Rescue Service added: ‘Southsea and Cosham crews were called at 6.07pm yesterday evening to a rescue from height incident on London Road, after a casualty had suffered a number of injuries jumping onto a roof above a shop.
‘HIWFRS gained access to the casualty and assisted with his removal from the roof.
‘The casualty was conscious and breathing, and treated by SCAS, HART and HEMS paramedics, who took him onto hospital.’