Księży Mostek [ˈkɕɛ̃ʐɨ ˈmɔstɛk] is a settlement in the administrative district of Gmina Płaska, within Augustów County, Podlaskie Voivodeship, in north-eastern Poland, close to the border with Belarus. It lies approximately 6 kilometres (4 mi) north-east of Płaska, 24 km (15 mi) north-east of Augustów, and 93 km (58 mi) north of the regional capital Białystok.