The accident happened at the junction of the Castlereagh and Upper Knockbreda roads around 9.45pm last night, Wednesday, September 27.
Sergeant Marion Curran said: “We are very keen to hear from anyone who saw this collision last night and could help with our investigation into what happened.
“A 19 year old man is currently in hospital with life-threatening injuries and a man in his 60s has been released from hospital following treatment for minor injuries.
“If you have any information that could help police, please contact us on the non-emergency number 101, quoting reference 1398 of 27/9/17.”
A 20 year old man who was arrested on suspicion of dangerous driving, causing grievous bodily injury, is currently in custody, helping police with their enquiries.