Hot springs in Japan

An onsen (温泉) is a Japanese hot spring that meets government standards — water must emerge naturally at 25°C or higher, or contain specific minerals like sodium or bicarbonate. Regular hot springs worldwide have no such legal definition. Many onsen are also bathing facilities (onsen ryokan or day-use sento) built around a natural source.

Traditional onsen require bathing naked, and tattoos are often prohibited at public facilities. However, mixed-gender rotenburo (outdoor baths) sometimes allow bathing suits, and private baths (kashikiri-buro) can be reserved for couples or families. Always check the specific facility's rules before visiting.

Autumn (October–November) and winter (December–February) are the most popular seasons. Soaking in an outdoor rotenburo while snow falls — called yuki-no-yu — is considered one of Japan's quintessential experiences. Summer is quieter and more affordable, though the contrast with the surrounding air is less dramatic.

Kyushu leads in volume — Beppu alone claims eight distinct spring types (the Beppu Hachito) and thousands of individual sources. Tohoku (Tōhoku), the Japanese Alps, and Hokkaido also have exceptionally dense onsen cultures with centuries of bathing tradition.