The Mangawhai Harbour is a natural harbour in New Zealand. It is a sandspit estuary on the south-eastern coast of the Northland Region. The townships of Mangawhai and Mangawhai Heads are on the harbour. The Mangawhai Government Purpose Wildlife Refuge Reserve covers the sandspit between the harbour and the ocean.

The Mangawhai Harbour is situated five kilometres south of Paepae-o-Tū / Bream Tail, a major headland in the Northland Region. The central section of the Mangawhai Harbour is called the Mangawhai Estuary. It is fed by a number of tributaries, including the Mangawhai River, Bob Creek and Tara Creek.

A large sand spit forms the mouth of the harbour. This sandspit is a 245 hectare nature reserve, known as the Mangawhai Government Purpose Wildlife Refuge Reserve.

The sandspit at the mouth of the harbour has spinifex and pingao, and is a home for many migratory bird species, such as Caspian terns, New Zealand fairy terns, Variable oystercatchers and New Zealand dotterels.

The Mangawhai Harbour is a part of the rohe of Te Uri-o-Hau, a tribal group either seen as an independent iwi, or as a hapū of Ngāti Whātua. The name refers to Te Whai, a historic rangatira of Ngāti Whātua, who shares his name with the Short-tail stingray, whai, found in the harbour. Te Whai had a fortified pā at Mangawhai Point, a central headland in the harbour. The harbour was an important connection between the east and west coasts of Northland, as it was a part of the Ōtamatea portage, a place which allowed waka to be hauled overland between the Mangawhai Harbour and the Kaipara Harbour, via the Kaiwaka River, Hakaru River and Otamatea River. 41 archaeological sites have been identified around the harbour.

The Mangawhai Harbour was purchased by the Crown in 1858, with European settlers using the harbour from 1859, primarily for the kauri logging and kauri gum industries. As these industries dwindled, dairy farming and sheep farming became the main industries around the harbour. Two European settlements developed around the harbour, Mangawhai to the south-west and Mangawhai Heads to the north.

In 1864, Three Brothers, a British schooner hit the sandbar at the mouth of the Mangawhai Harbour. Two passengers were washed overboard and drowned.

Sand mining began at the Mangawhai Harbour entrance pre 1940. In 1978 the collapse of sand dunes, believed to be caused by sand mining, closed the harbour for five and a half years. Commercial sand mining ceased in 2004.