The crash took place on the Eastern Road at 5pm on Sunday.
A dark grey Kia reportedly hit a 27-year-old pedestrian from Southsea, leaving him with serious injuries.
The man remains at QA Hospital, where he is being treated for his injuries, police have confirmed.
Now, a spokeswoman for Hampshire Constabulary has confirmed to Spotted Portsmouth that a 26-year-old woman from Waterlooville has been arrested on suspicion of drug-driving and causing grievous bodily harm with intent.
She is currently in police custody.
Officers are keen to speak to anyone who witnessed the collision. Anyone with information should call police on 101 quoting 44220030619.
As previously reported, the road was for several hours following the incident.
South Central Ambulance Service (Scas) said that two people were initially treated for injuries at the scene.
A spokeswoman for Scas said, “We sent to the scene two ambulance crews, a paramedic officer and the Hampshire and Isle of Wight Air Ambulance.
“They were assessing and treating two patients at the scene. Following treatment at the scene by our crews, one patient was taken by land ambulance to Queen Alexandra Hospital with serious injuries.”
The road closure was between Airport Service Road to the north and Tangier Road to the south, and it is thought the collision happened near the Burrfields Road junction.