The collision happened in Salvington Road, Durrington, at just after 4pm.
A spokesman for Adur and Worthing Police said: “We’re currently responding to a collision between a silver Ford Focus and a pedestrian, which occurred in Salvington Road, Durrington, about 4.12pm.
“A boy, aged six, sustained serious – potentially life-threatening – injuries, and is being taken to St George’s Hospital in London for treatment.
“A section of the road is likely to remain closed for some time as emergency services remain at the scene, and motorists are advised to seek alternative routes.
“Anyone who saw what happened, or who has dash cam or CCTV which may have captured the incident, is asked to contact us quoting Operation Radius.”