The Freshwater River is the longest river on Stewart Island / Rakiura, the third largest of New Zealand's islands. It arises close to the island's northwestern point, from which it is separated by a ridge, and flows southeastward through the Ruggedy Flat for 25 kilometres before reaching the Paterson Inlet on the island's central east coast. The freshwater is the longest New Zealand river not to be located in either of the country's two main islands.

The river's catchment dominates much of the island's north, forming a wide swampy basin that covers some 150 square kilometres (almost a tenth of the island's total area).

A tramping track runs along the lower reach of the river.