Hokkaido (Japanese: 北海道, Hepburn: Hokkaidō; pronounced [hok.kaꜜi.doː] ^ⓘ, lit. 'Northern Sea Circuit'; Ainu: Aynu Mosir, lit. 'Land of the Ainu') is the second-largest and northernmost of Japan's four main islands. Together with its surrounding islands, it comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu. The two islands are connected by railway via the Seikan Tunnel.

The largest city on Hokkaido is its capital, Sapporo, which is also its only ordinance-designated city. Sakhalin lies about 43 kilometres (27 mi) to the north of Hokkaidō. To the east and northeast are the Kuril Islands, which are administered by Russia. The four most southerly are claimed by Japan. Hokkaido's position on the northern end of the Japanese archipelago results in a colder climate, with the island seeing significant snowfall each winter. Despite the harsher climate, it serves as an agricultural breadbasket for many crops.

Hokkaido was formerly known as Ezo, Yezo, Yeso, or Yesso. Although Japanese settlers ruled the southern tip of the island since the 16th century, Hokkaido was primarily inhabited by the Ainu people. In 1869, following the Meiji Restoration, the entire island was annexed, colonized and renamed Hokkaido by Japan. Japanese settlers dispossessed the Ainu of their land and forced them to assimilate. In the 21st century, the Ainu are almost totally assimilated into Japanese society. As a result, the majority of Japanese people of Ainu descent have no knowledge of their heritage and culture.

When establishing the Development Commission, the Meiji government decided to change the name of Ezochi. Matsuura Takeshirō submitted six proposals, including names such as Kaihokudō (海北道) and Hokkaidō (北加伊道), to the government. The government eventually decided to use the name Hokkaidō, but decided to write it as 北海道, as a compromise between 海北道 and 北加伊道 because of the similarity with names such as Tōkaidō (東海道). According to Matsuura, the name was thought up because the Ainu called the region Kai. The kai element also strongly resembles the On'yomi, or Sino-Japanese, reading of the characters 蝦夷 (on'yomi as [ka.i, カイ], kun'yomi as [e.mi.ɕi, えみし]) which have been used for over a thousand years in China and Japan as the standard orthographic form to be used when referring to Ainu and related peoples; it is possible that Matsuura's kai was actually an alteration, influenced by the Sino-Japanese reading of 蝦夷 Ka-i, of the Nivkh exonym for the Ainu, namely Qoy or Gilyak pronunciation: [kʰuɣɪ].

In 1947, Hokkaidō became a full-fledged prefecture. The historical suffix 道 (-dō) translates to "prefecture" in English, ambiguously the same as 府 (-fu) for Osaka and Kyoto, and 県 (-ken) for the rest of the "prefectures". Dō, as shorthand, can be used to uniquely identify Hokkaido, for example as in 道道 (dōdō, "Hokkaido road") or 道議会 (Dōgikai, "Hokkaido Assembly"), the same way 都 (-to) is used for Tokyo. The prefecture's government calls itself the "Hokkaidō Government" rather than the "Hokkaidō Prefectural Government".

With the rise of indigenous rights movements, there emerged a notion that Hokkaido should have an Ainu language name. If a decision to change the name is made, however, whichever Ainu phrase is chosen, its original referent is critically different from the large geographical entity. The phrase aynumosir (アイヌモシㇼ) has been a preferred choice among Japanese activists. Its primary meaning is the "land of humans", as opposed to the "land of gods" (kamuymosir). When contrasted with sisammosir (the land of the neighbors, often pointing to Honshu or Japanese settlements on the southern tip of Hokkaido), it means the land of the Ainu people, which, depending on context, can refer to Hokkaido, although from a modern ethnolinguistic point of view, the Ainu people have extended their domain to a large part of Sakhalin and the entire Kuril Islands. Another phrase, yaunmosir (ヤウンモシㇼ), has gained prominence. It literally means the "onshore land", as opposed to the "offshore land" (repunmosir), which, depending on context, can refer to the Kuril Islands, Honshu, or any foreign country. If the speaker is a resident of Hokkaido, yaunmosir can refer to Hokkaido. Yet another phrase, akor mosir (アコㇿモシㇼ) means "our (inclusive) land". When used by Hokkaido Ainus, it can refer to Hokkaido or Japan as a whole.

During the Jomon period the local culture and the associated hunter-gatherer lifestyle flourished in Hokkaidō, beginning over 15,000 years ago. In contrast to the island of Honshu, Hokkaidō saw an absence of conflict during this time period. Jomon beliefs in natural spirits are theorized to be the origins of Ainu spirituality. About 2,000 years ago, the island was colonized by Yayoi people, and much of the island's population shifted away from hunting and gathering towards agriculture.

Ebetsu Kofun Cluster is a group of mounds dating from the latter half of the 8th century to the middle of the 9th century, and were surveyed by Juichi Goto in 1931. Along with the Kofun, Warabidetoh (蕨手刀), iron sword, Magatama, Sueki were discovered. Ebetsu Kofun is known to be evidence for the existence of Wajin (和人) and the trading between Hokkaido and Honshu before Feudal Japan.