Castronuño is a municipality in the province of Valladolid, within the autonomous community of Castile and León, Spain. It lies in the northern part of the country, in the region historically known as Old Castile.
According to the 2004 census carried out by Spain's National Statistics Institute (INE), the municipality had a population of 1,049 inhabitants. As a municipality, Castronuño covers both the town itself and the surrounding area under a single local administration.