The Douglas River, formerly known as the Twain, is a river of the West Coast of New Zealand's South Island. Its source is high in the Southern Alps, five kilometres south of Mount Sefton, and its upper reaches are fed by water from the Douglas Glacier. It flows west for 18 kilometres, joined by runoff from the Horace Walker Glacier, before joining the waters of the Karangarua River. The Douglas River's entire course is within Westland Tai Poutini National Park. The river and glacier are named after Charles Edward Douglas, a 19th-century explorer and mountaineer.
The New Zealand Department of Conservation maintains a backcountry hut at the junction of the Douglas and Horace Walker rivers.
Coordinates: 43°40′29″S 169°50′26″E / 43.67472°S 169.84056°E