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".
Ashya King's parents told the Daily Mail they fled the UK after being told NHS treatments would leave him with life-long disabilities.
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.