Osiek Piaseczny [ˈɔɕɛk pjaˈsɛt͡ʂnɨ] is a village in the administrative district of Gmina Zawidz, within Sierpc County, Masovian Voivodeship, in east-central Poland. It lies approximately 5 kilometres (3 mi) east of Zawidz, 21 km (13 mi) south-east of Sierpc, and 98 km (61 mi) north-west of Warsaw.