The Clutha River / Mata-Au is the second longest river in New Zealand and the longest in the South Island. It flows south-southeast 338 kilometres (210 mi) through Central and South Otago from Lake Wānaka in the Southern Alps to the Pacific Ocean, 75 kilometres (47 mi) south west of Dunedin. Gold is in abundance in the Clutha River and its surrounding areas. It is the highest volume river in New Zealand, and has a discharging mean flow of 614 cubic metres per second (21,700 cu ft/s).

The Clutha River played a prominent role in both the Māori and European history of the area. Rivers and valleys were the main transport system used by local Māori to access the interior of the South Island. The 1860s Otago gold rush resulted in the production of approximately 240 tonnes of gold, which was found in the Clutha catchment. It has the biggest catchment and outflow in New Zealand. About 6% of all water in the South Island is discharged by the Clutha River alone. It has a mean discharge of approximately 500 m³ (18,000 cu ft) and a catchment area of around 22,000 km² (8,500 sq mi) and is an economically significant river for the country. The Clutha River encompasses two hydropower stations, which provide 14% of the country's hydropower generation capacity.

The Clutha River drains the high mountains of the Southern Alps in the west and passes through a complex topographic system of basins and ranges towards the east before reaching into the Pacific Ocean. A majority of the topographical features of the Clutha River catchment area are a direct result of the late Cenozoic and active tectonic processes that are occurring in southern South Island due to deformation along the nearby plate boundary, defined by the Alpine Fault.

The river is known for its scenery, gold-rush history, and swift turquoise waters. A river conservation group, the Clutha Mata-Au River Parkway Group, is working to establish a regional river parkway, with a trail, along the entire river corridor.

The Māori name for the Clutha River is the Mata-Au (sometimes shortened to Matau), meaning 'surface current'. Early settlers sometimes spelled the Māori name as "Matou" and "Matua-a", and pronounced it "Mattoo". Māori also referred to the Clutha River as Maranuku. The name Matau is widely used to refer to one of the two distributaries of the Clutha close to its mouth (the other is the Koau).

The first appearance of a European name for the Clutha River / Mata-Au was the Molyneux River (/mɒlɪnˌəʊ/); its mouth was named by Captain James Cook after his sailing master, Robert Molineux. The name is also applied to the small settlement of Port Molyneux. Early maps show Moulineux Harbour in its original spelling, but later maps indicate the harbour's name was written as "Molyneux", rather than "Moulineux".

The river is now commonly known as the Clutha, which comes from Cluaidh, the Scots Gaelic name for the River Clyde in Scotland, which runs through Glasgow.

The official name for the river has been Clutha River / Mata-au since the Ngāi Tahu Claims Settlement Act 1998, a landmark Treaty of Waitangi settlement, which added dual names to approximately 90 geographic features throughout the South Island to recognise the "equal and special significance" of both the English and Māori names.