The woman, aged 19, was a passenger in a car heading towards Rotherham town centre when her condition suddenly deteriorated and the driver stopped on Fitzwilliam Road and called for an ambulance.
Police said they were called to the scene at around 12.10pm today, following a report from Yorkshire Ambulance Service that the woman was unresponsive to medial assistance.
The woman was removed from the vehicle for treatment but was sadly pronounced dead at the scene.
South Yorkshire Police said her family have been informed and are being supported by officers.
Her death is currently being treated as unexplained, and enquiries are ongoing.