bay of the South Pacific Ocean and geographic region, northern North Island, New Zealand, formed when the sea flooded an old river valley system. The bay has a shoreline of 500 miles (800 km) and about 150 islands. It opens to the sea through an 11-mile- (18-kilometre-) wide passage flanked by Brett Cape on the east and Wiwiki Cape on the west.

The first European to enter the inlet was Captain James Cook in 1769. Three years later the French explorer Marion du Fresne founded a short-lived settlement on Moturoa Island. In 1814 the Reverend Samuel Marsden established the first permanent European settlement in New Zealand at Rangihoua. A centre for whalers during the 1830s, the bay was the site of the signing of the Treaty of Waitangi in 1840 between Great Britain and native Maoris. The bay is now a resort area. It offers historic sites such as the Waitangi Treaty House, as well as deep-sea fishing, hunting, and scenic beauty. Principal towns are Russell, Paihia, and Kerikeri. Urapukapuka is a tourist centre. A deepwater port of entry for overseas shipping is located at Opua.