Kosmaczewo [kɔsmaˈt͡ʂɛvɔ] is a village in the administrative district of Gmina Zawidz, within Sierpc County, Masovian Voivodeship, in east-central Poland. It lies approximately 6 kilometres (4 mi) south of Zawidz, 19 km (12 mi) south-east of Sierpc, and 99 km (62 mi) north-west of Warsaw.