The South Island, also officially named Te Waipounamu, is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman Sea, and to the south and east by the Pacific Ocean. The South Island covers 150,437 square kilometres (58,084 sq mi), making it the world's 12th-largest island. At low altitude, it has an oceanic climate.

The South Island is shaped by the Southern Alps which run along it from north to south. They include New Zealand's highest peak, Aoraki / Mount Cook at 3,724 metres (12,218 ft). The high Kaikōura Ranges lie to the northeast. The east side of the island is home to the Canterbury Plains while the West Coast is famous for its rough coastlines such as Fiordland, a very high proportion of native bush and national parks, and the Fox and Franz Josef Glaciers. The main centres are Christchurch and Dunedin. The economy relies on agriculture and fishing, tourism, and general manufacturing and services.

As it is 32% larger than the North Island, the South Island is sometimes nicknamed the "mainland" of New Zealand. It is home to 23 percent of New Zealand's 5.1 million inhabitants. After the 1860s gold rushes in the early stages of Pākehā (European) settlement of the country, the South Island had the majority of the European population and wealth. The North Island's population overtook the South Island's in the early 20th century, with 56% of the New Zealand population living in the North Island in 1911. The drift north of people and businesses continued throughout the twentieth century.