The man was holding onto the rear bumper of a box truck, heading southbound on Amsterdam Avenue, when the vehicle hit a bump.
The skateboarder lost his grip on the truck, fell to the ground and hit his head on the pavement.
He was taken to St. Luke's Hospital in critical condition.
The box truck did not stop. At this point, it appears the driver did not know he had a person holding onto the back.
Police have interviewed a witness who observed the entire incident.