The South Island is one of the two major islands of New Zealand, the other being the North Island. The Māori name for the South Island, Te Wai Pounamu, meaning "The Water/s of Greenstone" (greenstone being jade), possibly evolved from Te Wāhi Pounamu which means "The Place Of Greenstone". The island is also known as Te Waka a Māui which means "Māui's Canoe".

In the 19th century, some maps named the South Island as Middle Island or New Ulster, and the name South Island or New Leinster was used for today's Stewart Island/Rakiura.

It has an area of 151,215 square km (58,093 square miles), making it the world's 12th-largest island. Along its west coast runs the mountain chain of the Southern Alps with Aoraki/Mount Cook being the highest point, 3,754 m (12,316 ft) above sea level.

The South Island is often called the Mainland (somewhat humorously) because it existed first according to Māori legend, and is larger than the North Island, but it has only around a million of the four million Kiwis in New Zealand.

Historically as well as more recently, the South Island Independence movement has promoted financial and administrative self-government for the South Island.