Admiralty Bay is a large indentation in the northern coast of New Zealand's South Island. It lies close to the northernmost mainland point of the Marlborough Sounds, immediately to the south of D'Urville Island.
The bay is one of the larger of numerous bays in the crenellated coast of the sounds, being 10 kilometres wide at its mouth and extending eight kilometres south. The peninsula into which it cuts is almost bisected by the bay, with a narrow isthmus only some 900 metres in width lying between the bay's southernmost extent and Hallam Cove to the south. To the northeast, the bay is open to the waters of Cook Strait, but to the west a narrow and treacherous stretch of water French Pass is the only maritime access.
Admiralty Bay is closely associated with Pelorus Jack, a dolphin which became widely known in New Zealand in the first few years of the 20th century. Pelorus Jack was noted for meeting and escorting ships through French Pass, and is possibly the first individual sea creature protected by law in any country.