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Nearly 3,000 hot spring destinations are scattered across Japan, from Hokkaido in the north to Kyushu in the south. Hot spring legends appear in Japanese mythology, Heian-period aristocrats traveled for therapeutic bathing, Edo-era commoners enjoyed long bathing journeys, and even today, hot spring towns welcome over 100 million overnight guests every year. Few countries in the world have grown this close to their hot waters.
And yet, many of us enjoy onsen without truly knowing what an onsen is. What exactly qualifies as an onsen? Why have the Japanese felt such a deep affinity for these waters? And why are hot springs so beloved here?
To deepen your understanding of these questions, let us unravel the world of Japanese onsen culture.
From the geological mechanics of hot springs to the cultural background that has captivated the Japanese for over a thousand years, this guide is intended to make your next onsen trip even more rewarding.
To be called an onsen in Japan, a hot spring must meet the criteria set by a specific law: the Hot Springs Law (Onsen-hō), enacted in 1948 (Showa 23). Administered by the Ministry of the Environment, the law defines an onsen as follows.
Warm water, mineral water, water vapor, and other gases (excluding natural gas whose main component is hydrocarbon) that emerge from underground.
In addition, a spring must satisfy at least one of the two criteria below to be classified as an onsen.
What surprises many is that even cold water below 25°C (77°F) can be officially recognized as an onsen if it contains the required mineral components. For example, cold mineral springs rich in carbon dioxide are sometimes enjoyed as “natural carbonated springs” (tennen tansansen). The popular image of onsen as simply hot water is not the whole story.
Among onsen, those considered suitable for therapeutic use are categorized as therapeutic hot springs (ryōyōsen). Separately from the Hot Springs Law, the Ministry of the Environment’s Guidelines for Hot Spring Analysis set stricter temperature and component requirements for this category. The names of spring qualities you see at onsen towns, such as “simple hot spring” (tanjun onsen), “sulfur spring” (iō-sen), and “sodium bicarbonate spring” (tansansuiso-en sen), correspond to these therapeutic categories. It is the difference in mineral composition that creates the variations in color, scent, texture, and reputed benefits.
The characteristics of each spring quality will be explored in detail in the second article of this series: the Japanese Onsen Spring Quality Guide.
The relationship between the Japanese and hot springs reaches back further than written history.
One of the oldest onsen legends is associated with Dōgo Onsen in Ehime Prefecture. According to a fragment of the Iyo no Kuni Fudoki (an ancient regional gazetteer), in the age of myth, the deity Ōkuninushi-no-Mikoto (a major god in Japanese mythology) is said to have bathed the ailing Sukunahikona-no-Mikoto (a small deity associated with medicine and hot springs) in the waters of Dōgo. Sukunahikona-no-Mikoto recovered at once and danced upon a stone. That stone, now called Tama-no-Ishi (Jewel Stone), is enshrined on the north side of the present Dōgo Onsen Honkan.
The Nihon Shoki (Japan’s oldest official chronicle, compiled in 720) records imperial visits (gyōkō, a formal trip by an emperor) to the “Iyo Hot Waters” by Emperor Jomei, Empress Saimei, and Prince Naka-no-Ōe (later Emperor Tenji). Beyond mythology, actual emperors traveled to enjoy these waters.
Arima Onsen in Hyogo Prefecture also has ancient records. The Nihon Shoki describes an imperial visit in 631 CE (the 3rd year of Emperor Jomei’s reign). At the heart of the hot spring town stands Tōsen Shrine, which enshrines three deities: Ōnamuchi-no-Mikoto, Sukunahikona-no-Mikoto, and Kumano Kusumi-no-Mikoto. The first two are traditionally honored as the discoverers of the hot spring.
Equally fascinating are the “White Heron Legend” (Hakuro Densetsu) and the “Crane’s Bath Legend” (Tsuru-no-yu Densetsu) found across Japan. In these stories, a wounded white heron, crane, or yatagarasu (a mythical three-legged crow) heals itself by soaking in a hot spring, and villagers who witness the scene discover the waters. Similar legends are passed down at Gero Onsen, Yamashiro Onsen, Kaminoyama Onsen, and many other onsen towns. The idea that animals revealed the power of the hot springs has become part of each town’s identity.
In the medieval period (from 1185), therapeutic bathing (tōji) by samurai and Buddhist monks flourished. In 1276, during the Kamakura period, Ippen Shōnin (the founder of Ji-shū, a sect of Pure Land Buddhism) is said to have opened the mushiyu (steam bath) at Kannawa Onsen in Beppu.
Court nobles and shogunate officials sometimes avoided traveling to the springs themselves and instead had the water transported in tubs and barrels to their residences. This luxurious practice was known as kumiyu (transported hot spring water).
Onsen culture truly spread to commoners after the start of the Edo period (from 1603). The development of post roads made travel across Japan far easier.
The Tokugawa shogunate institutionalized o-kumiyu, a practice of transporting hot spring water in barrels from places such as Atami and Kusatsu all the way to Edo Castle (today’s Tokyo) for the shogun’s use. Shoguns including Tokugawa Iemitsu and Tokugawa Yoshimune are said to have been particularly fond of these waters.
Commoners applied to their local lord for permission to travel through a formal request known as tōji-negai (a request for therapeutic bathing leave), then made long stays of about three weeks for therapeutic bathing. Twenty-one days, counted as three cycles of seven, had been the traditional unit of tōji since medieval times.
In the late Edo period (from 1781), the Onsen Banzuke (hot spring rankings) were published. Modeled after sumo’s east-west ranking sheets (banzuke), they listed hot spring destinations across Japan, along with their reputed benefits and the number of days it took to travel from Edo. This guide accelerated the onsen travel boom. Travelers on pilgrimages to Ise or Kompira often chose hot spring towns over regular post stations for a single night’s stay, a practice called ichiya-tōji (one-night therapeutic bathing).
In the Meiji era (from 1868), the spread of railway networks transformed onsen towns. Mountain hot springs that had previously been reachable only by determined walkers gradually became popular destinations for the general public.
After World War II, the boom in group travel during the high-growth era supported onsen towns. Today, they remain a centerpiece of both domestic and inbound tourism, drawing visitors much as they did more than a thousand years ago.
A detailed introduction to historic hot springs will appear in the fourth article of this series: a guide to Japan’s most famous onsen.
There are many different ways and places to enjoy an onsen in Japan. You can stay overnight at a ryokan, drop in for a day-use bath, tour the local public bathhouses of a hot spring town, or simply soak your feet. Japan offers a wide range of options for every occasion and preference.
An onsen ryokan is a uniquely Japanese form of accommodation that combines lodging, dining, and bathing. Guests rest on futons in tatami rooms, wear yukata (light cotton robes) inside the inn, and enjoy kaiseki (multi-course) dinners and breakfasts either in their rooms or at a dining hall. A one-night stay with two meals included is the standard, and you can enter the inn’s baths as many times as you like at no extra cost.
Hotels, by contrast, are modern accommodations centered on Western-style rooms. Many offer restaurant dining, large public baths, and entertainment facilities.
These facilities are for bathing only, without an overnight stay. They range from small public bathhouses to large complexes equipped with rest areas, restaurants, and massage rooms.
Many hotels and ryokan also offer day-use bathing for visitors who only want to enjoy the onsen without staying the night.
These are public onsen bathhouses managed by the local residents of a hot spring town. In contrast to a ryokan’s uchiyu (indoor bath), they are also called sotoyu (outdoor public baths). Because they are run as community infrastructure, the entrance fee is often lower than that of a ryokan’s day-use bath.
For example, Kinosaki Onsen in Hyogo Prefecture is famous for treating the entire town as a single inn and is known for its culture of bathhouse-hopping among seven sotoyu.
Nozawa Onsen in Nagano has 13 sotoyu, all maintained by a local community cooperative known as yu-nakama (literally, “bath companions”).
As these examples show, sotoyu are common in onsen towns with long histories.
A foot bath, or ashiyu, is a simple bathing facility where you soak from the calves down while fully clothed. Many hot spring towns offer foot baths in front of stations, at sightseeing spots, and at accommodations, and most are free to use.
Beyond onsen, Japan has another form of public bathhouse deeply rooted in local communities: the sentō. Before private bathrooms became common in Japanese homes, sentō supported daily life, and they remain a familiar part of the urban landscape today.
Sentō can be divided into two main types based on scale and function.
| Category | Public Bathhouse Law classification | Governing law | Supervising ministry | Main water source | Price regulation |
|---|---|---|---|---|---|
| Onsen | - | Hot Springs Law | Ministry of the Environment | Naturally occurring spring water | None |
| Sentō | General public bathhouse | Public Bathhouse Law | Ministry of Health, Labour and Welfare | Heated tap water | Yes (prefectural governors set the regulated price) |
| Super sentō | Other public bathhouse | Public Bathhouse Law | Ministry of Health, Labour and Welfare | Heated tap water | None |
Sentō lie at the heart of Japan’s bathing culture. In the past, they existed in nearly every region of Japan, including Tokyo and Osaka.
Yet despite their cultural importance, most sentō are not onsen. Their water is heated tap water and does not meet the Hot Springs Law’s definition of an onsen.
Japan’s onsen culture has shaped a remarkable variety of bath designs and bathing styles. “Going into an onsen” can mean very different things depending on the type of bath.
The indoor bath (uchiyu) is a tub set inside the bathhouse. It is the most basic form of onsen and can be enjoyed even on rainy or snowy days.
The open-air bath (rotenburo) is set outdoors. Whether nestled along a mountain valley, perched on a cliff above the sea, or surrounded by snowy scenery, its appeal lies in the openness of soaking in the midst of nature.
The utaseyu is a traditional bathing style in which hot spring water falls from a height like a waterfall onto the shoulders or lower back, producing an effect similar to a modern massage. It is most often found at onsen towns blessed with abundant water flow.
The mushiyu is an ancient Japanese bathing style that uses the heat of an onsen. The most famous example is the "Kannawa Mushiyu" at Beppu Onsen, said to have been founded in 1276 by Ippen Shōnin, the founder of Ji-shū (a sect of Pure Land Buddhism). In a small stone room with hot spring steam rising through the floor, bathers lie in yukata on a bed of medicinal sekishō (a sweet-flag herb).
The sunayu is a therapeutic style in which the body is buried up to the neck in coastal sand warmed by hot spring heat. Ibusuki Onsen in Kagoshima is the most representative example. The hot spring water rising along the shore warms the sand, creating a natural form of thermal therapy that has been enjoyed since ancient times. Beppu Onsen also has a sand bath, known there as a suna-mushi-buro (sand steam bath).
Onsen are part of Japan’s history, culture, and daily life.
The fact that hot water rises from underground reveals the existence of a powerful heat source beneath the islands. It is proof that Japan is one of the most volcanically active regions in the world, home to about 7% of the world’s active volcanoes.
For nearly a thousand years, the Japanese have lived alongside this land and these waters.
The same hot springs that, in legend, healed the gods also soothed the wounds of samurai and monks, and welcomed Edo-period travelers at the end of their long journeys. The form may have changed over the centuries, but the time spent loosening body and mind in hot water still flows through everyday Japanese life today.
Where will your next onsen trip take you? When you next sink into a Japanese hot spring, take a moment to remember the long history that those waters have traveled through.
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