Oshima Peninsula

Lore & Background

The Oshima Peninsula forms the southernmost extremity of Hokkaidō, Japan’s northernmost main island. Its terrain is predominantly mountainous, with most human settlements confined to flat lowland areas. The peninsula is defined by a southern fork into two distinct landforms: the Matsumae Peninsula, which points southwest, and the Kameda Peninsula, which points southeast. These two projections face the Tsugaru and Shimokita Peninsulas of Honshū across the Tsugaru Strait. The region is geologically active, containing several volcanoes such as Mount E and Hokkaido Koma-ga-take, and numerous onsen hot springs. The Ōnuma Quasi-National Park is located here. Two cities, Hakodate and the smaller adjacent Hokuto, anchor the peninsula, alongside many small fishing and agricultural towns. A notable historical structure is Matsumae Castle, the northernmost castle in Japan and the only castle on Hokkaido. The climate is humid continental (Dfa/Dfb) across most of the area, shifting to oceanic (Cfb) along the Matsumae Peninsula coast. The hardiness zone is unusually high for an east-coast Northern Hemisphere location at this latitude, reaching up to 8b on the Matsumae. Ecologically, the peninsula is separated from the rest of Hokkaido by the Kuromatsunai Depression, an area of intensive agriculture that acts as a natural northern boundary. The peninsula supports an active population of Ussuri brown bears, which have been indiscriminately hunted for over 120 years due to historical damage to livestock and crops.

Reader's Guide

The Oshima Peninsula forms the southernmost part of Hokkaidō, Japan’s northernmost main island. Its precise northern boundary is debated; a broad definition places it southeast from Ishikari Bay across the Ishikari Plain, while a narrower one connects Suttsu on the Sea of Japan to Oshamambe on Uchiura Bay, encompassing the Oshima and Hiyama subprefectures. The peninsula’s southern end splits into two distinct forks: the Matsumae Peninsula pointing southwest and the Kameda Peninsula pointing southeast. These face the Tsugaru and Shimokita Peninsulas of Honshū across the Tsugaru Strait. The terrain is mountainous, with most settlements confined to flat lowland areas. Active volcanoes, including Mount E and Hokkaido Koma-ga-take, are present, alongside numerous onsen hot springs. The main urban centers are Hakodate and the smaller adjacent city of Hokuto, supported by many small fishing and agricultural towns. Ecologically, the Kuromatsunai Depression—a zone of intensive agriculture—separates the peninsula from the rest of Hokkaidō, forming a natural northern boundary. This area hosts an active population of Ussuri brown bears, which have caused significant damage to livestock and crops, leading to over 120 years of indiscriminate hunting. The peninsula’s climate is humid continental (Dfa/Dfb), except along the Matsumae Peninsula coast, where it is oceanic (Cfb). Its hardiness zone is notably high for an east-coast Northern Hemisphere location at this latitude, reaching up to 8b on the Matsumae Peninsula. The Ōnuma Quasi-National Park lies within the peninsula. Its historical and cultural significance is underscored by Matsumae Castle, the northernmost castle in Japan and the only castle on Hokkaidō.

Did You Know?

• The Matsumae clan's jurisdiction was initially limited to the Wajin territory on the Oshima Peninsula in southwest Hokkaido.
• The Matsumae clan's economic foundation relied on exclusive trade with the Ainu people of Ezo, not on agriculture.
• In 1604, Tokugawa Ieyasu granted Matsumae Yoshihiro a black seal that gave the clan exclusive trading rights with the Emishi (Ainu people).
• By 1834, the castle town of Matsumae had evolved into a thriving urban center with a populace exceeding 10,000 residents.
• Matsumae Castle is the northernmost castle in Japan.