The two kanji that make up the word onsen (温泉) mean “hot” and “source”, respectively. While this is the fundamental definition of a hot spring, to be officially recognised as an onsen in Japan a hot spring must meet one of two criteria:
- The water must be at least 25°C at the source
- It must meet one of 19 criteria relating to the mineral content of the water. These criteria are stipulations such as: metaboric acid (HBO2) greater than 5mg per 1kg, hydrogen ion greater than 1mg per 1kg, and so on
Public bathing at the onsen is by no means a recent phenomenon in Japanese culture. References in the Kogiji—the oldest extant chronicle in Japan—show that Japanese have been bathing in the hot springs for well over a thousand years, and there are many historical accounts of feudal lords having their own favourite onsen spots (kakushiyu) where they may have let their samurai bathe after battle (the onsen has long been known for its healing qualities).
The hot springs have a strong association with regional Japan and there are countless onsen towns dotted up and down the country. From Tokyo, Hakone is the most famous (and easily accessible) onsen region, but there are many others within an hour on the Shinkansen.
The facilities listed on this website are day onsen—hot spring facilities that do not offer overnight accommodation. They are closer to day retreats in that the services offered include massages services, hot stone spas, relaxation areas, restaurants, and even hair salons. Prices vary from ¥500 to ¥2,500 for use of the baths with an additional charge for the other services. For those travelling on a budget who want to experience the bathing culture in Japan the day onsen are an excellent option—an overnight stay at a ryokan would typically cost between ¥15,000 to ¥25,000 per person per night in comparison. And because the day onsen are dedicated hot springs, the bathing area (and thus the number and variety of baths) is usually far greater than at the ryokan, which are only catering for a limited number of staying guests.