The Thylacine (pronounced /ˈθaɪləsaɪn/, or in Australia /ˈθaɪləsiːn/, also /ˈθaɪləsɨn/) (binomial name: Thylacinus cynocephalus; Greek for "dog-headed pouched one") was the largest known carnivorous marsupial of modern times. It is commonly known as the Tasmanian Tiger (because of its striped back), the Tasmanian Wolf, and colloquially the Tassie (or Tazzy) Tiger or simply the Tiger. Native to continental Australia, Tasmania and New Guinea, it is thought to have become extinct in the 20th century. It was the last extant member of its genus, Thylacinus, although several related species have been found in the fossil record dating back to the early Miocene.

The Thylacine became extinct on the Australian mainland thousands of years before European settlement of the continent, but it survived on the island state of Tasmania along with several other endemic species, including the Tasmanian Devil. Intensive hunting encouraged by bounties is generally blamed for its extinction, but other contributory factors may have been disease, the introduction of dogs, and human encroachment into its habitat. Despite its official classification as extinct, sightings are still reported.

Like the tigers and wolves of the Northern Hemisphere, from which it obtained two of its common names, the Thylacine was an apex predator. As a marsupial, it was not related to these placental mammals, but because of convergent evolution it displayed the same general form and adaptations. Its closest living relative is thought to be either the Tasmanian Devil or Numbat.

The Thylacine was one of only two marsupials to have a pouch in both sexes (the other being the Water Opossum). The male Thylacine had a pouch that acted as a protective sheath, protecting the male's external reproductive organs while running through thick brush.

Although the Thylacine is considered extinct, many people believe the animal still exists. Sightings are regularly claimed in Tasmania, other parts of Australia and even in the Western New Guinea area of Indonesia, near the Papua New Guinea border. The Australian Rare Fauna Research Association reports having 3,800 sightings on file from mainland Australia since the 1936 extinction date, while the Mystery Animal Research Centre of Australia recorded 138 up to 1998, and the Department of Conservation and Land Management recorded 65 in Western Australia over the same period. Independent Thylacine researchers Buck and Joan Emburg of Tasmania report 360 Tasmanian and 269 mainland post-extinction 20th century sightings, figures compiled from a number of sources. On the mainland, sightings are most frequently reported in Southern Victoria.

Some sightings have generated a large amount of publicity. In 1973, Gary and Liz Doyle shot ten seconds of 8mm film showing an unidentified animal running across a South Australia road. However, attempts to positively identify the creature as a thylacine have been impossible due to the poor quality of the film. In 1982 a researcher with the Tasmania Parks and Wildlife Service, Hans Naarding, observed what he believed to be a Thylacine for three minutes during the night at a site near Arthur River in northwestern Tasmania. The sighting led to an extensive year-long government-funded search. In January 1995, a Parks and Wildlife officer reported observing a Thylacine in the Pyengana region of northeastern Tasmania in the early hours of the morning. Later searches revealed no trace of the animal. In 1997, it was reported that locals and missionaries near Mount Carstensz in Western New Guinea had sighted Thylacines. The locals had apparently known about them for many years but had not made an official report. In February 2005 Klaus Emmerichs, a German tourist, claimed to have taken digital photographs of a Thylacine he saw near the Lake St Clair National Park, but the authenticity of the photographs has not been established. The photos were not published until April 2006, fourteen months after the sighting. The photographs, which showed only the back of the animal, were said by those who studied them to be inconclusive as evidence of the Thylacine's continued existence.

In 1983, Ted Turner offered a $100,000 reward for proof of the continued existence of the Thylacine. However, a letter sent in response to an inquiry by a Thylacine-searcher, Murray McAllister, in 2000 indicated that the reward had been withdrawn. In March 2005, Australian news magazine The Bulletin, as part of its 125th anniversary celebrations, offered a $1.25 million reward for the safe capture of a live Thylacine. When the offer closed at the end of June 2005 no one had produced any evidence of the animal's existence. An offer of $1.75 million has subsequently been offered by a Tasmanian tour operator, Stewart Malcolm. Trapping is illegal under the terms of the Thylacine's protection, so any reward made for its capture is invalid, since a trapping licence would not be issued.