Police were called to the Hillside area at Stithians Lake in Carnmenellis, Redruth, Cornwall, about 20:50 BST on Tuesday following reports of a crash.
The driver of the Ford Fiesta was taken to the Royal Cornwall Hospital where she later died.
The road was closed for more than five hours. No other vehicles were involved, Devon and Cornwall Police said.
More on this story, and other Cornwall news
A woman has died after her car plunged into a lake after a crash.