Castrillo de Onielo is a municipality in the province of Palencia, in the autonomous community of Castile and León, Spain. It is located in the northern part of the country, within the broad plains of the Castilian plateau that characterize much of this region.
The municipality is small and rural, with a population of 101 inhabitants recorded in the 2018 census by Spain's National Statistics Institute (INE). Like many villages in the province of Palencia, it is part of a sparsely populated agricultural landscape.