Castilruiz is a municipality in the province of Soria, within the autonomous community of Castile and León, Spain. It is located in the eastern part of the province, in the region of the Iberian System known as the Tierras Altas de Soria.
The municipality has a small population, recorded at 150 inhabitants in the 2022 census by Spain's National Statistics Institute (INE). Like many rural settlements in the Soria highlands, it is characterised by a dispersed, sparsely populated landscape and a traditional agricultural economy.
Castilruiz is one of numerous small municipalities that make up the province of Soria, one of the least densely populated provinces in Spain and in the wider European Union.