Castejón de Sos (in Benasquese, Castilló de Sos) is a municipality in the province of Huesca, in the Aragon region of Spain. It lies in the northeastern part of the country, within the comarca of Ribagorza in the Pyrenean foothills.
According to the 2004 census (INE), the municipality had a population of 731 inhabitants. The town is known by two names: the Castilian form Castejón de Sos and the local Benasquese form Castilló de Sos, reflecting the linguistic heritage of the surrounding Benasque valley area.