Knajka is a stream in the Cieszyn County, Silesian Voivodeship, Poland, in the historical region of Cieszyn Silesia. It is a left tributary of the Vistula, which it enters just south of Strumień. Its length is either 17 kilometres (11 mi) or 19 kilometres (12 mi). It flows through Ogrodzona, Kostkowice, Dębowiec, Knaj and Bąków. Numerous fish ponds are located alongside of it.
The name of the river is derived from a word knieja denoting a wild, dense forest.