Whanganui Island is the largest of a small group of islands at the entrance of Coromandel harbour in the Hauraki Gulf, off the coast of New Zealand's North Island.

The island, which is privately owned, is 2.83 square kilometres. Much of it is farmed, but there are some forested areas.

The island was the capital of NZ for 2 weeks in the 1840's.

It was owned by an American settler until the signing of the Treaty of Waitangi when he lost legal title to the island as all titles passed to the British crown.

Today Woolshed Bay on the protected southern shore is a popular over night mooring spot for recreational cruising yachts and launches from Auckland.