Hokkaidō (Japanese kanji: 北海道; hiragana: ほっかいどう; lit. "northern sea circuit") is the northernmost of Japan's four main islands. Unlike the other three Japanese main islands which are divided into multiple prefectures (similar to a county), the entire island of Hokkaidō comprises just one prefecture, with its capital being the major city of Sapporo.
Whereas the history of Japan itself spans well over a thousand years on its three other islands, large-scale Japanese colonization of what is now Hokkaidō only began in the 19th century. Before this time, Japan called this island Ezo, (kanji: 蝦夷; hiragana: えぞ; lit. "foreigner") and it was the primary homeland of the Ainu people, now a small minority of Hokkaidō's population. During the Japanese feudal era that serves as the setting of some Teenage Mutant Ninja Turtles backstories, Ezo was still effectively a foreign land.
In the IDW continuity story arc Karai's Path, a remote woodland in Hokkaidō is the location of Takeshi Tatsuo's vault, where centuries ago he hid away a powerful supernatural sword, the Kira no Ken, until Oroku Karai entered the vault and claimed it.