In Māori mythology, taniwha (Māori pronunciation: [ˈtaniɸa]) are large supernatural beings that live in deep pools in rivers, dark caves, or in the sea, especially in places with dangerous currents or deceptive breakers (giant waves). They may be considered highly respected kaitiaki (protective guardians) of people and places, or in some traditions as dangerous, predatory beings, which for example would kidnap women to have as wives.

Linguists have reconstructed the word taniwha to Proto-Oceanic *tanifa, with the meaning "shark species". In Tongan and Niuean, tenifa refers to a large dangerous shark, as does the Samoan tanifa; the Tokelauan tanifa is a sea-monster that eats people. In most other Polynesian languages, the cognate words refer to sharks or simply fish.

Some anthropologists have stated that the taniwha has "analogues that appear within other Polynesian cosmologies". Moʻo or Moho, reptilian deities which can be seen in Hawaiian lore, strongly share characteristics with Taniwha as being both protector and harmful to men, shape-shifting, and so on.

At sea, a taniwha often appears as a whale or a large shark such as southern right whale or whale shark; compare the Māori name for the great white shark: mangō-taniwha. In inland waters, they may still be of whale-like dimensions, but look more like a gecko or a tuatara, having a row of spines along the back. Other taniwha appear as a floating log, which behaves in a disconcerting way. Some can tunnel through the earth, uprooting trees in the process. Legends credit certain taniwha with creating harbours by carving out a channel to the ocean. Wellington's harbour, Te Whanganui-a-Tara, was reputedly carved out by two taniwha. The petrified remains of one of them turned into a hill overlooking the city. Lake Waikaremoana in Te Urewera Ranges area of Wairoa District was also reputedly carved out by taniwha. Other taniwha allegedly caused landslides beside lakes or rivers.

Taniwha can either be male or female. The taniwha Āraiteuru is said to have arrived in New Zealand with the early voyaging canoes and her eleven sons are credited with creating the various branches of the Hokianga Harbour.

Most taniwha have associations with tribal groups; each group may have a taniwha of its own. The taniwha Ureia, depicted on this page, was associated as a guardian with the Māori people of the Hauraki district. Many well-known taniwha arrived from Hawaiki, often as guardians of a particular ancestral canoe. Once arrived in Aotearoa, they took on a protective role over the descendants of the crew of the canoe they had accompanied. The origins of many other taniwha are unknown.

When accorded appropriate respect, taniwha usually acted well towards their people. Taniwha acted as guardians by warning of the approach of enemies, communicating the information via a priest who was a medium; sometimes the taniwha saved people from drowning. Because they lived in dangerous or dark and gloomy places, the people were careful to placate the taniwha with appropriate offerings if they needed to be in the vicinity or to pass by its lair. These offerings were often of a green twig, accompanied by a fitting incantation. In harvest time, the first kūmara (sweet potato) or the first taro was often presented to the taniwha.

Arising from the role of taniwha as tribal guardians, the word can also refer in a complimentary way to chiefs. The famous saying of the Tainui people of the Waikato district plays on this double meaning: Waikato taniwha rau (Waikato of a hundred chiefs).