Emergency services were called to the unclassified Cruden Bay to Gask road near North Teuchan, Aberdeenshire at 10.50pm on Saturday.
The collision involved a white Vauxhall Corsa, with a 17-year-old being the only person involved.
He was taken to Aberdeen Royal Infirmary with serious injuries, and Police Scotland shut the route for a number of hours.
Road Policing Sergeant Colin Matheson said: “We would like to hear from anyone who saw a white Vauxhall Corsa at the time of the collision or in the area beforehand.
“Anyone with any information should contact Police Scotland on 101 quoting reference number 5302 of May 19.”