Olszewka [ɔlˈʂɛfka] is a village in the administrative district of Gmina Jednorożec, within Przasnysz County, Masovian Voivodeship, in east-central Poland. It lies approximately 10 kilometres (6 mi) north of Jednorożec, 25 km (16 mi) north-east of Przasnysz, and 111 km (69 mi) north of Warsaw.