Ainu (Ainu: アイヌ・イタㇰ, Aynu itak; Japanese: アイヌ語 Ainu-go) is one of the Ainu languages, spoken by members of the Ainu ethnic group on the northern Japanese island of Hokkaidō.

Until the twentieth century, Ainu languages were also spoken throughout the southern half of the island of Sakhalin and by small numbers of people in the Kuril Islands. All but the Hokkaidō language are extinct, with the last speaker of Sakhalin Ainu having died in 1994; and Hokkaidō Ainu is moribund, though there are ongoing attempts to revive it.

Ainu has no generally accepted genealogical relationship to any other language family. For the most frequent proposals, see Ainu languages.

Ainu is a moribund language, and has been endangered for at least the past few decades. Most of the 25,000 – 200,000 ethnic Ainu in Japan speak only Japanese. In the town of Nibutani (part of Biratori, Hokkaidō) where many of the remaining native speakers live, there are 100 speakers, out of which only 15 used the language every day in the late 1980s.

The Ainu (アイヌ^?), also called Aynu, Aino (アイノ), and in historical texts Ezo (蝦夷), are indigenous people or groups in Japan and Russia. Historically, they spoke the Ainu language and related varieties and lived in Hokkaidō, the Kuril Islands, and much of Sakhalin. Most of those who identify themselves as Ainu still live in this same region, though the exact number of living Ainu is unknown. This is due to confusion over mixed heritages and to ethnic issues in Japan resulting in those with Ainu backgrounds hiding their identities. In Japan, because of intermarriage over many years with Japanese, the concept of a pure Ainu ethnic group is no longer feasible. Official estimates of the population are of around 25,000, while the unofficial number is upward of 200,000 people.

Recent research suggests that Ainu culture originated in a merger of the Okhotsk and Satsumon cultures. In 1264, Nivkh people reported to the Yuan Dynasty of the Mongol Empire that Ainu invaded the land of Nivkh, resulting in battles between Ainu and the Yuan Dynasty. Active contact between the Wajin (the ethnically Japanese) and the Ainu of Ezochi (now known as Hokkaido) began in the 13th century. The Ainu were a society of hunter-gatherers, who lived mainly by hunting and fishing, and the people followed a religion based on phenomena of nature.