The 34-year-old male victim, who has not been named, suffered fatal injuries when a silver Vauxhall Signum struck a tree. The driver fled the scene.
No other vehicle was involved in the accident on the B7015 at Sheephousehill Road on the outskirts of Fauldhouse, West Lothian, at 7:45pm on Tuesday.
The driver of the car had left the scene by the time emergency services arrived.
Police said a member of the public had alerted emergency services about the collision.
A spokeswoman for Lothian and Borders fire service said firefighters used hydraulic cutting gear to release the trapped passenger from the vehicle.
The man was then taken by ambulance to Edinburgh Royal Infirmary.
It is believed the vehicle was heading west before it crashed about 100m east of Sheephousehill Road’s junction with Breichwater Place.
Sergeant Gary Taylor, of Lothian and Borders Police, said: “We are appealing for any witnesses to the accident, or anyone who saw the car prior to 7:45pm, to contact us.”
The officer added: “We are also keen to speak to the driver of the car who left the scene prior to the arrival of the emergency services.”