Tupaia (also spelled Tupaea or Tupita; c. 1725 – 20 December 1770) was a Tahitian Polynesian navigator and arioi (a kind of priest), originally from the island of Ra'iatea in the Pacific Islands group known to Europeans as the Society Islands. His remarkable navigational skills and Pacific geographical knowledge were used by Lt. James Cook, R.N. when he took him aboard HMS Endeavour as guide on its voyage of exploration to Terra Australis Incognita. Tupaia travelled with Cook to New Zealand, acting as the expedition's navigator to the Polynesian Māori, and Australia. He died in December 1770 from a shipborne illness contracted when Endeavour was docked in Batavia for repairs ahead of its return journey to England.

Tupaia was born at Ha'amanino Harbour on Ra'iatea around 1725 and became a leading ariori priest for the Taputapuatea marae. Tupaia was trained in the fare-'ai-ra'a-'upu, or schools of learning, about the origin of the cosmos, genealogies, the calendar, proverbs and histories. He was also taught how to be a star navigator. His knowledge included island lists, including their size, reef and harbor locations, whether they were inhabited, and if so, the name of the chief and any food produced there. More importantly, his skills would include the bearing of each island, the time to get there, and the succession of stars and islands to follow to get there. These islands included the Society Islands, the Austral Islands, the Cook Islands, plus Samoa, Tonga, Tokelau and Rotuma.

Bora Bora warriors invaded Ra'iatea around 1763, wounding Tupaia and forcing him to flee to Tahiti, where he sought protection from the Papara chief Amo and his wife Purea. Tupaia soon became their advisor and high priest, and eventually Purea's lover. Tupaia befriended Samuel Wallis during his observation of a solar eclipse, and then Joseph Banks during the 1769 Transit of Venus observed from Tahiti. After which, Tupaia "attached himself to the British," according to Anne Salmond. Tupaia was also an artist, and ten watercolors of his survive.

"The Captain refuses to take [Tupaia] on his own account, in my opinion sensibly enough, the government will never in all human probability take any notice of him. I therefore have resolved to take him ...

I do not know why I may not keep him as a curiosity, as well as some of my neighbours do lions and tigers at a larger expense than he will probably ever put me to; the amusement I shall have in his future conversation and the benefit he will be to this ship as well as what he may be if another should be sent to these seas, will I think fully repay me."

Tupaia joined Endeavour in July 1769 when it passed his home island of Ra'iatea in the outward voyage from Plymouth. He was welcomed aboard at the insistence of Sir Joseph Banks, the Cook expedition's official botanist, on the basis of his evident skill as a navigator and mapmaker: when asked for details of the region Tupaia drew a chart showing all 130 islands within a 2,000 miles (3,200 km) radius and was able to name 74 of them. Banks welcomed the Raiatean's interest in travelling with Endeavour to England because of his usefulness for the ship as well as the envisaged conversation and amusement (possibly even genuine friendship according to Australian researcher Vanessa Smith). It was also hoped that he could be presented as an anthropological curiosity in England. As Cook at first refused to allow Tupaia to join the expedition for financial reasons, Banks agreed to be responsible for the Raiatean's welfare and upkeep while on board.

As Cook intended to spend several weeks in the Society Islands before heading south, Tupaia assisted the expedition as an interlocutor and interpreter with local tribes. He also worked closely with Banks in compiling an account of Tahiti and its inhabitants. On August 15, 1769, Tupaia began to work on a Chart of the Pacific Ocean in collaboration with Cook, Banks, and several of Cook's officers.

Older research summarized by Joan Druett assumed that Tupaia's own voyaging experience was limited. It holds that Tupaia had navigated from Ra'iatea in short voyages to 13 islands shown on the resulting map. He had not visited western Polynesia, as since his grandfather's time the extent of voyaging by Raiateans had diminished to the islands of eastern Polynesia. His grandfather and father had passed to Tupaia the knowledge as to the location of the major islands of western Polynesia and the navigation information necessary to voyage to Rotuma, Samoa and Tonga. It was also assumed that Cook was less pleased than Banks with Tupaia's evident navigational skills, resolving instead to rely on his own exploration of the region.

More recent research challenged the view that Tupaia's travels in the wider region were limited, and questioned Cook's failing appreciation of Tupaia as misinterpretations of the source material. In an extended reading of Tupaia's Map, Lars Eckstein and Anja Schwarz propose that Tupaia had detailed navigational knowledge that extended throughout the Polynesian triangle (with the probable exception of only New Zealand). The chart he drew for James Cook in August 1769 shows interconnected voyaging routes ranging from Rotuma west of Samoa, via Samoa and Tonga, the southern Cook Islands and the Austral Group, Mangareva and Pitcairn all the way to Rapa Nui. A second major composite route leads from Tahiti through the Tuamotu Group to the Marquesas Group and on to Oahu in Hawai'i. Tupaia invented a cartographic system for Cook and his men which located a northern bearing from any island he drew in the centre of his Chart (marked by the word 'avatea', this is '[the sun at] noon'). This allowed him to translate his own wayfinding knowledge for island-to-island voyages into the logic and terms of Cook's compass. The Admiralty manuscript of James Cook's journal indicates that Tupaia told Cook that he himself (or his ancestors) travelled to most islands drawn on the Chart excepting only Rotuma (north of Fiji) and Oahu in Hawai'i.