Chatham Island is by far the largest island of the Chatham Islands group, in the south Pacific Ocean off the eastern coast of New Zealand, named after the survey ship HMS Chatham which discovered the island in 1791. It covers an area of 900 km².

The geography of the roughly T-shaped island is dominated by three features: two bays and a lagoon.

More than half of the west coast of Chatham is taken up by the deep indentation of Petre Bay. The island's main settlement of Waitangi is located on Waitangi Bay, a small indentation in Petre Bay's southern coast.

On the east coast is the even larger Hanson Bay, which stretches for the entire 35 kilometre length of the island.

The ten kilometre width of the island, between these two bays, is entirely illusory. Much of it is taken up by the large Te Whanga Lagoon, which drains to the see in the southern half of Hanson Bay. This lagoon covers about 180 km², and drains several small rivers that rise in the hills at the island's south end.