Castillazuelo is a municipality in the province of Huesca, in the autonomous community of Aragon, Spain. It is located in the Somontano de Barbastro comarca, a district in the foothills of the Pyrenees in northern Aragon.
The municipality is small and sparsely populated. According to the 2018 census conducted by Spain's National Statistics Institute (INE), Castillazuelo had a population of 169 inhabitants.