The 16-year-old boy was travelling along Kingston Road yesterday evening when he was struck by a black Vauxhall Corsa.
He was injured in the crash, and was taken to Southampton General Hospital.
Another rider, a 14-year-old boy, was also hurt in the collision and received pain to his leg.
Officers are investigating the circumstances behind what happened.
A spokesman from Hampshire Constabulary said: ‘Police were called at 5.48pm yesterday evening following reports of a collision on Kingston Hill in Portsmouth, between a vehicle and two cyclists.
‘One of the cyclists, a 16-year-old boy from Portsmouth, suffered a minor injury and was taken to Southampton General Hospital.
‘Kingston Road and New Road were closed while emergency services dealt with the incident, but were re-opened a short while later.
‘No arrests were made, but enquires remain ongoing to ascertain exact circumstances of the incident.’