The 79-year-old was driving a blue Toyota Avensis, which collided with the road verge and some small trees at Tongland at about 12 noon.
Emergency services attended and she was taken by ambulance to Dumfries and Galloway Royal Infirmary, where police say she remains in a critical condition.
Constable John Jamieson of the Roads Policing Unit said: “We are investigating the cause of the crash, however indications are that the driver may have suffered a medical episode at the time.”
