Police said the 26-year-old victim was in Gamble Road, Buckland, when he was attacked at around 9.10pm last night. He fled to Derby Road, North End, and called for help.
Today police said the victim suffered a ‘laceration to the neck’ - while the assailant drove off.
A 24 year-old man has been arrested on suspicion of attempted murder and is still in custody.
The victim is still in Queen Alexandra Hospital in Cosham undergoing treatment. His injuries are not thought to be life-threatening, police said.
A Hampshire police spokesman said: ‘We are seeking witnesses who may have dash cam footage that may assist our enquiries after a 26 year-old man was seriously assaulted in Portsmouth last night.
‘At around 9.10pm the victim, who had been walking along Gamble Road, was approached by a man who had gotten out of a blue vehicle.
‘The man assaulted the victim, leaving him with a laceration to his neck, and drove off.
‘The victim called for help when he got to Derby Road and remains at Q+A hospital for treatment. His injuries are not life threatening.’
Police are patrolling the area today.
Anyone who saw what happened or has dash-cam footage should call 101 quoting 44200379979.
Alternatively, you can call Crimestoppers anonymously on 0800 555 111.