And police are appealing for information about the collision on the A710 near Southwick.
It happened at about 9 pm and involved a grey Vauxhall Corsa which left the road and hit a tree.
Emergency services rushed to the scene and the car’s three occupants were taken to Dumfries and Galloway Royal Infirmary.
Officers said the 20-year-old man is in a critical condition, while two 17-year-olds have minor injuries.
A force spokesman said: “An investigation is underway to establish the circumstances of the crash. The road was closed for several hours as officers examined the scene.
“We are appealing for anyone who may have been in the area around the time of the incident or who may have seen the Vauxhall Corsa on the road prior to the collision to contact us. In particular, we are asking motorists with dash cams to check their footage as it may have captured images which could assist the investigation.”
Call 101 if you can help.