The Arnold River is a river on the West Coast of New Zealand's South Island. It is the outflow of Lake Brunner, which it links with the Grey River at Stillwater. The Arnold River flows northwest for 20 kilometres (12 mi), joining the Grey immediately above the town of Brunner, some 15 kilometres (9 mi) from the Tasman Sea. It is a popular spot for whitewater kayaking and trout fishing.