Biernik [ˈbjɛrnik] is a village in the administrative district of Gmina Puszcza Mariańska, within Żyrardów County, Masovian Voivodeship, in east-central Poland. It lies approximately 2 kilometres (1 mi) south of Puszcza Mariańska, 11 km (7 mi) south-west of Żyrardów, and 53 km (33 mi) south-west of Warsaw.