South Yorkshire Police said the 16-year-old girl and 19-year-old man were passengers in a black VW Polo which collided with a wall and overturned on White Lane, Chapeltown, Sheffield.
A 17-year-old boy driving the car suffered minor injuries.
The accident happened near the entrance to Hesley Wood Scout Activity Camp at around 9.55pm on Friday March 24, but police only released details today.
The teenagers suffered serious injuries and were taken to hospital for treatment. They were later discharged from hospital.
Officers investigating believe that a red Ford Fiesta was travelling behind the Polo before the collision and that the Polo may have collided with a silver vehicle shortly before as well.
It is understood that the Polo was travelling out of Chapeltown towards Barnsley.
Police would like to speak to anyone who was either involved in the collision or who saw what happened.
Call police on 101, quoting incident number 1239 of 24 March 2017.