The accident happened on the A705 near Seafield, West Lothian, at around 11.05am on Christmas Eve when the Vauxhall Astra came off the road.
Driver Louis Kefferty, 20, died from his injuries in Edinburgh Royal Infirmary, where his 18-year-old passenger Lauren White is still being treated. Police said she is likely to be in hospital "for some time" but her injuries are not life-threatening.
In a Facebook post she said that her boyfriend had saved her life and made sure she was safe before tending to himself.
She wrote: "Yesterday I lost the most precious person in my life, he has been there for me no matter what and it has left a hole in my heart that will never mend."
Meanwhile, an elderly female driver has died after a two-vehicle crash on the A90, around a mile south of Fraserburgh, around 3pm on Christmas Eve. She has not yet been named.
A separate crash in Aberdeenshire has also left two people in hospital. Emergency services were called to the scene of the head-on collision on the A96 Huntly to Glens of Foudland road at around 12.10pm on Christmas Eve.
The collision involved a silver BMW 3 Series being driven east towards Inverurie by a 65-year-old woman and a black Audi A3 being driven west by a 58-year-old man. There were no passengers in either vehicle.
The woman was airlifted to Aberdeen Royal Infirmary by an RAF Lossiemouth search and rescue helicopter and remains in a serious condition.
The man was taken to the same hospital by ambulance and is being treated for a broken collarbone and other minor injuries.
Police are appealing for witnesses in all three crashes.