Nasielsk [ˈnaɕelsk] is a small town in Masovian Voivodeship, Poland. It is located on a major Warsaw-Gdańsk rail line and serves as a junction, with an additional connection to Sierpc. In the past, the name of the town was spelled in different forms: Nasilzco, Nosidlsk, Nosylsk, Nosydlsk, Nosielsk, Nosselia, Nosidlsko, Nasidlsko, Nosilsko, Nasilsko, Nasylsco.