Aldeamayor de San Martín 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 Iberian Peninsula, in the region traditionally known as Old Castile.
According to the 2014 census conducted by Spain's National Statistics Institute (INE), the municipality had a population of 5,060 inhabitants. The town is situated close to the provincial capital, Valladolid, in a part of the country characterised by a continental climate and a landscape of plains and farmland.