Osiek [ˈɔɕɛk] is a village in the administrative district of Gmina Joniec, within Płońsk County, Masovian Voivodeship, in east-central Poland. It lies approximately 3 kilometres (2 mi) west of Joniec, 12 km (7 mi) east of Płońsk, and 53 km (33 mi) north-west of Warsaw.

This gentry village ("wieś szlachecka", i.e., owned by a noble) became part of the Zakroczymska land [pl] in the second half of the XVI. Between 1975 and 1998, this place belonged to the Ciechanów Voivodeship.