Alcazarén is a town and municipality in the Province of Valladolid, within the autonomous community of Castile and León, Spain. It has a population of 702 and covers an area of 48 km², with a perimeter of 33.51 km.
The town lies in the Tierra de Pinares region, approximately 35 km south of Valladolid, the provincial capital. Its name derives from the Arabic "al-qasrayn," meaning "the two castles."