A 17-year-old boy is in Cairns hospital with life-threatening head injuries after falling off his bike while being towed alongside a sedan.

Queensland Police said the boy was riding his bike along Deep Acres Drive in Palm Cove, north of Cairns, just after 10pm on Tuesday night.

He was holding on to a Toyota Aurion sedan while riding, with an 18-year-old man known to the boy driving the car.

As the car came toward Argentea Boulevard the boy fell from the bike, landing heavily on the road.

He was rushed to Cairns Hospital and the Forensic Crash Unit is investigating.