Emergency services were called to the River Tavy near Roborough in Devon at 9:55am yesterday after reports of a car in the water.
It is not known when the car entered the water, but the incident is not believed to have been weather-related.
Cops said the woman was pulled out of the vehicle by firefighters and received treatment at the scene, but later died in hospital.
The woman’s next of kin have been informed, and her death is not being treated as suspicious.
Devon and Cornwall Police said: “Search and rescue teams, assisted by the Coastguard helicopter, were involved in continued searches of the river and its surrounds to ensure there were no other casualties.
“The searches have now concluded and officers are content that no other person was in the vehicle.
“The car was secured by the fire service and has since been recovered.”