Tiritiri Matangi Island is located in the Hauraki Gulf of New Zealand, 3.4 km (2.1 mi) east of the Whangaparāoa Peninsula in the North Island and 30 km (19 mi) north east of Auckland. The 2.2 km² (1 sq mi) island is an open nature reserve managed by the Supporters of Tiritiri Matangi Incorporated, under the supervision of the Department of Conservation and is noted for its bird life, including takahē, North Island kōkako and kiwi. It attracts between 30,000 and 32,000 visitors a year, the latter figure being the maximum allowed by the Auckland Conservation Management Strategy.

The name, Māori for "tossed by the wind", is often popularly shortened to Tiritiri. Māori mythology considers the island to be a float of an ancestral fishing net.

The island is located on the Hibiscus Coast, to the east of Whangaparāoa Peninsula, and is composed of ancient greywacke rock.

The first people to settle on the island were Māori of the Kawerau iwi. Later, members of the Ngāti Pāoa moved to the island, like the Kawerau partly for shark fishing until about 1700, when the Kawerau regained control and remained until forced to retreat to Waikato in 1821 when Hongi Hika attacked from the north. There were two pā, Tiritiri Matangi Pā located to the north of Hobbs Bay, and Papakura Pā, to the north-west of the island.

European (Pākehā) settlers arrived in the early 19th century. In 1841 Ngāti Pāoa sold the land to the crown as part of the Mahurangi Block. When the Kawerau returned, friction ensued as both peoples had a claim to the island. In 1867 the Māori Land Court awarded title to the Crown.

A lighthouse was constructed near the southern end in 1864, and remains in operation. In 1956, a xenon light source was fitted to the lighthouse, creating the most powerful light-beam achieved at the time by a New Zealand lighthouse. It had an output of 11 million candle-power and a range of 58 nautical miles, making it one of the most powerful lights in the world; most lights shone for 27 nautical miles.

The island was farmed from 1863 to 1971 by the Hobbs family, who also owned land on the peninsula. When the lease expired, management was then vested in the Hauraki Gulf Maritime Park Board.

While originally forested, the island developed into pasture by the mid-20th century. From 1984, the island has been the focus of a wide-scale native forest regeneration project, where over 250,000 native plants have been propagated on the island. The island was chosen as a unique and protected place to provide a public window for rare New Zealand native birds on the edge of a large city and it also lacked introduced predators such as mustelids which were present on the mainland. At that time, although the island was devoid of suitable habitat and food sources, the hope was that native forest would regenerate naturally. It became apparent that natural afforestation was happening very slowly because a forest can only grow at its margins, and the island was covered mostly with dense grass and bracken fern. A plan was formulated to establish a nursery to collect cuttings and seed in order to expand the small pockets of forest habitat that were left in some of the valleys. Pōhutukawa was chosen as the main tree as it would eventually provide perches and roosts for birds who would then excrete the seed of the fruits that they had been eating, which would then germinate around the pōhutukawa.