Emergency services were called to the A158 Burgh bypass at Gunby at 6.30am on Saturday after a car left the road and collided with a tree.
The driver, a 19 year-old man from Burgh Le Marsh, suffered serious injuries and was taken to Boston Pilgrim Hospital. He was later transferred to the Queens Medical Centre in Nottingham.
The road was reopened at 11.45am. Anyone with information about the collision, involving a red BMW 318i, should contact the Collision Witness Hotline on 01522 558855.