The tūī (Prosthemadera novaeseelandiae) is a medium-sized bird native to New Zealand. It is blue, green, and bronze coloured with a distinctive white throat tuft (poi). It is an endemic passerine bird of New Zealand, and the only species in the genus Prosthemadera. It is one of the largest species in the diverse Australasian honeyeater family Meliphagidae, and one of two living species of that family found in New Zealand, the other being the New Zealand bellbird (Anthornis melanura). The tūī has a wide distribution in the archipelago, ranging from the subtropical Kermadec Islands to the sub-Antarctic Auckland Islands, as well as the main islands.

Europeans first encountered the tūī in 1770 at Queen Charlotte Sound on the north coast of New Zealand's South Island during Captain James Cook's first voyage to the Pacific Ocean. Specimens were brought back to England and an engraving of a tūī by the English naturalist Peter Brown, which he called "The New Zeland creeper", was published in 1776. The tūī was seen on all three of Cook's voyages. Cook's account of his second voyage to the Pacific was published in 1777 and included a description and an illustration of the tūī. He used the names "poly-bird" and "poe-bird". He praised the bird: "The flesh is most delicious, and was the greatest luxury the woods afforded us." In 1782 the English ornithologist John Latham included the tūī as the "poë bee-eater" in his book A General Synopsis of Birds. Latham described a specimen in the Leverian Museum in London. No author had introduced a scientific name, but when in 1788 the German naturalist Johann Friedrich Gmelin revised and expanded Carl Linnaeus's Systema Naturae, he included the tūī with a short description, coined the binomial name Merops novaeseelandiae and cited the publications by Brown, Cook and Latham. The tūī is now the only species placed in the genus Prosthemadera that was introduced in 1840 by the English zoologist George Gray. The genus name combines the Ancient Greek prosthema meaning "appendage" with dera meaning "neck".

Two subspecies are recognised:

The bird's name comes from the Māori language. The plural is tūī in modern New Zealand English, or ngā tūī in Māori usage; some speakers still use the '-s' suffix to produce the Anglicised form tūīs to indicate plurality, but this practice is becoming less common. For many years the prevailing spelling was tui without the macrons that indicate long vowels, but spelling Māori loanwords with macrons is now common in New Zealand English. The International Ornithologists' Union (IOC), which has a policy of not using accents, lists Tui as the bird's English name. Early European colonists called it the parson bird or mocking-bird but these names are no longer used.

The closest living relative to tūī is the New Zealand bellbird; genetic analysis indicates its ancestor diverged from a lineage that gave rise to the New Zealand and Chatham bellbirds around 5 million years ago. The cladogram below shows this relationship:

The tūī is a large honeyeater, 27 to 32 cm (11–13 in) in length. The Chatham Islands subspecies is larger on average than the nominate subspecies, and heavier. Males tend to be heavier than females. Nominate males weigh between 65–150 g (2.3–5.3 oz), and females 58–105 g (2.0–3.7 oz). Males of the Chatham subspecies are 89–240 g (3.1–8.5 oz) and females 89–170 g (3.1–6.0 oz).

At first glance the bird appears completely black except for a small tuft of white feathers at its neck and a small white wing patch, causing it to resemble a parson in clerical attire. On closer inspection (see image) it can be seen that tūī have brown feathers on the back and flanks, a multicoloured iridescent sheen that varies with the angle from which the light strikes them, and a dusting of small, white-shafted feathers on the back and sides of the neck that produce a lacy collar.

Tūī are native to New Zealand, and are found throughout the country, particularly the North Island, the west and south coasts of the South Island, Stewart Island / Rakiura and the Chatham Islands, where an endangered sub-species particular to these islands exists. Other populations live on Raoul Island in the Kermadecs, and in the Auckland Islands (where, with the New Zealand bellbird, it is the most southerly species of honeyeater). Traditionally, Māori ate tūī that had been preserved in calabashes or gourds. Populations have declined considerably since European settlement, mainly as a result of widespread habitat destruction and predation by mammalian invasive species.^{[citation needed]}