The girl was riding her horse along Joan Croft Lane at around 5pm yesterday (February 9), when a car is believed to have driven past the animal too fast, causing it to bolt.
The teenager fell from the horse and was taken to Sheffield Children's Hospital with serious head injuries.
The car involved is reported to be a white Seat Leon with dark tinted windows and dark wheels.
South Yorkshire Police are investigating the incident and want to speak to anyone who may have any information.
Detectives are keen to trace any CCTV footage and speak to any garage owners who have had similar cars in for damage repair.
Anyone with information can call police on 101, quoting incident number 981 of February 9.