Koski Duże [ˈkɔski ˈduʐɛ] is a village in the administrative district of Gmina Perlejewo, within Siemiatycze County, Podlaskie Voivodeship, in north-eastern Poland. It lies approximately 6 kilometres (4 mi) south-east of Perlejewo, 20 km (12 mi) north-west of Siemiatycze, and 75 km (47 mi) south-west of the regional capital Białystok.