Police were called to the accident scene on the A7, around half-a-mile north of Galashiels, around 9pm yesterday (Monday).
A red Seat was travelling south when it collided with a northbound black car of the same model, a statement said.
The 20-year-old male driver of the red car was taken to hospital, where medical staff have described his condition as critical.
The driver of the black car was uninjured.
The A7 was closed for several hours for investigation works and police are looking to trace the driver of a third car, only described as black, which was travelling infront of the red Seat shortly before the collision.
Sergeant Jill Kirkpatrick said: “Enquiries carried out so far have established that another black car, make currently unknown, was in front of the red Seat prior to the collision and I would ask the driver of that vehicle to contact us as they may have information which could assist our enquiries.
“I would appeal to anyone who was on the A7 around the time of the incident to contact us with any information or dashcam footage as it could prove vital in our investigation.”