The incident happened at approximately twenty past eight last night at Ballydrehid, just outside Cahir.
A 19 year old man-was taken to the South Tipperary General Hospital in Clonmel -with serious injuries.
The occupants of the second vehicle were uninjured.
Members of the Garda Scenes of Crime Team are carrying out a forensic examination of the scene.
The road is closed at Ballydrehid and is expected to remain closed until later this morning. Local diversions are in place.
