Gacnik (Polish pronunciation: [ˈɡat͡sɲik]) is a village in the administrative district of Gmina Brusy, within Chojnice County, Pomeranian Voivodeship, in northern Poland. It lies approximately 3 kilometres (2 mi) north-east of Brusy, 26 km (16 mi) north-east of Chojnice, and 78 km (48 mi) south-west of the regional capital Gdańsk.

For details of the history of the region, see History of Pomerania.