Kotan Onsen sits on the eastern shore of Lake Kussharo in eastern Hokkaidō, and features a stone open-air bath built almost at the same level as the lake surface. The name “Kotan” comes from the Ainu word for “settlement,” and the site is maintained and managed by local volunteers.

The spring is a sodium-bicarbonate spring—often called a “beauty bath”—and is free and open 24 hours a day. It’s closed only on scheduled cleaning days (Tuesdays and Fridays from May through October; Fridays from November through April). Management is handled by nearby residents and souvenir‐shop owners. The tub is essentially mixed-gender, though a large central rock gently divides the two sides; swimwear or a thin “yukata” towel is recommended.

Because the bath is right at lake level, soaking here feels like you’re part of Lake Kussharo itself. In winter (late November through early April), swans glide by just meters away, and at dusk the lake often turns to gold—a truly spectacular view.