The collision happened at Aghnatrisk Road in Culcavy, Hillsborough, Co Down, on Saturday at around 5.30 pm.
The teenager sustained head and leg injuries and was taken to the Royal Victoria Hospital.
Local police have appealed for information or witnesses to the crash.
Insp Jim McGrillen said: “We would like to hear from anyone who saw the incident involving the cycle and the Audi car, or saw them on the road beforehand.
“The incident occurred about a mile from the Culcavy Road/Harry’s Road junction.
“Please call Lisburn station on the 101 non-emergency number and the quote the reference 990 of August 8.”