Castrejón de la Peña 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 province, within the comarca of La Peña, a transitional area between the Meseta and the Cantabrian Mountains.
According to the 2018 census by Spain's National Statistics Institute (INE), the municipality had a population of 360 inhabitants. Like many rural municipalities in this part of Castile and León, it is sparsely populated and covers a dispersed area that includes several small settlements alongside the main village.
The surrounding landscape is characterised by the foothills and moorland typical of the Palencia mountain region, with agriculture and livestock forming the traditional economic base of the area.