The Selwyn River is located in the South Island of New Zealand. The Maori name for the river is Waikirikiri but this name is rarely used.

The river has its source in the foothills of the Southern Alps and flows east for 80 kilometres before emptying into Lake Ellesmere, to the south of Banks Peninsula.

For much of its course the river flows through wide shingle channels. In drought years, the river can be lost beneath this bed, and appear to dry up completely. This can happen as close to its outflow as where it is crossed by State Highway 1, at the settlement of Selwyn, only 20 kilometres from Ellesmere.