Ashya King is being treated in the Materno Infantil children's hospital in the Spanish city of Malaga, according to a tweet from the local police chief, Ignacio Cosido.
Mr Cosido wrote that a police patrol had found the five-year-old and congratulated them on their "good work".
The head of the police force which applied for a European Arrest warrant against the parents of Ashya King has now urged their release.
A British mother whose son's life was saved by proton therapy has told ITV that she defied NHS doctors' advice to pursue the treatment.
The parents of Ashya King said they took him out of a hospital to seek proton beam treatment which is not available under the NHS.