Prostyń [ˈprɔstɨɲ] is a village in the administrative district of Gmina Małkinia Górna, within Ostrów Mazowiecka County, Masovian Voivodeship, in east-central Poland. It lies approximately 6 kilometres (4 mi) south-west of Małkinia Górna, 16 km (10 mi) south of Ostrów Mazowiecka, and 84 km (52 mi) north-east of Warsaw. The village has a population of 840.