The Tasman Sea is the large body of water between Australia and New Zealand, approximately 2,000 kilometres (1,200 mi) across. It extends 2,800 km (approx.) from north to south. It is a south-western segment of the South Pacific Ocean. The sea was named after the Dutch explorer Abel Janszoon Tasman, the first recorded European to encounter New Zealand and Tasmania. The British explorer Captain James Cook later extensively navigated the Tasman Sea in the 1770s as part of his first voyage of exploration.

The Tasman Sea is commonly referred to in both Australia and New Zealand as The Ditch; for example, crossing the ditch means going to Australia from New Zealand or vice versa. In Māori, the Tasman Sea is called Te Tai-o-Rehua.