The woman died after the car she was travelling in went into a ditch at Dreenan, Ballybofey, at about 04:25 BST. She was originally from the Ballybofey area.
The driver, a 19-year-old man, has been taken to hospital where he is being treated for minor injuries.
The road has been closed while forensic officers carry out an investigation. The police have appealed for witnesses.
A 19-year-old woman has been killed in a car crash in County Donegal.