Sussex Police said the boy, who was not from the area, suffered “serious injuries” after falling from a cliff in Ovingdean – a small village in the east of Brighton and Hove.
Emergency services were called to the cliff face at 5pm on Saturday night to a report of a person “falling from a height”.
The injured boy, who is thought to have been on a visit to the East Sussex coast, was airlifted to hospital, where he remains.
The force said that enquiries into the boy’s fall are ongoing.
The 10-year-old and his family are receiving support from officers.
Sussex Police said in a statement: “Police responded to a report of a boy falling from height in Ovingdean, at about 5pm on Saturday September 24.
“The boy was found to have suffered serious injuries and was airlifted to hospital, where he remains critically ill. He is not local to the area and was visiting Brighton & Hove.
“Ovingdean roundabout was closed whilst the boy was bought to safety. The roundabout re-opened just before 10pm.
“We thank the public for their patience during the closure. The boy and his family are receiving support from officers and enquiries are continuing.”