Siennica [ɕɛnˈnit͡sa] is a village in the administrative district of Gmina Nasielsk, within Nowy Dwór County, Masovian Voivodeship, in east-central Poland. It lies approximately 3 kilometres (2 mi) south of Nasielsk, 18 km (11 mi) north-east of Nowy Dwór Mazowiecki, and 43 km (27 mi) north of Warsaw.