Łopacin [wɔˈpat͡ɕin] is a village in the administrative district of Gmina Sońsk, within Ciechanów County, Masovian Voivodeship, in east-central Poland. It lies approximately 6 kilometres (4 mi) south-west of Sońsk, 14 km (9 mi) south of Ciechanów, and 64 km (40 mi) north of Warsaw.