The emergency services were called to the scene on Cowbridge Road East, Canton, at about 15:50 BST.
South Wales Police said the girl was taken to the city's University Hospital of Wales with suspected head injuries.
The road was closed in both directions, but had reopened by 19:30 and There have been no arrests.
The Welsh Ambulance Service sent a rapid response vehicle and an emergency ambulance to the scene.
A six-year-old girl has been taken to hospital after being hit by a vehicle near Cardiff city centre.