Onsen Water Types: Reading the Signs

Not every hot spring is alike. Learn the main onsen water types, how to read the signs and analysis sheets, and how to choose a bath you will enjoy.

Published October 17, 2026 at 9:47 PM JST | By Tokyo Paul

Quick answer: Onsen water types are named for the main minerals dissolved in the spring, such as simple, chloride, sulfur, iron and acidic. The analysis sheet at a bath lists the spring quality, written as sensitsu, so you can learn what you are soaking in.

Hot springs are among the best-loved experiences in Japan, and the best onsen towns are famous for good reason. But once you start visiting different baths, you notice that no two are quite the same. One bath is crystal clear, another milky blue, another rusty gold. One smells like a match just struck, another has barely any scent at all. Those differences come from the minerals dissolved in the water, and Japan has a handy system for describing them. This guide shows you the main onsen water types, how to read the signs at the bath, and how to pick a spring that suits your trip.

What Makes a Spring an Onsen

In Japan, the word onsen has a legal meaning as well as a cultural one. Under the Hot Springs Act, onsen includes hot water, mineral water, and steam or gas from underground that meets set criteria, such as a minimum temperature or a minimum level of certain dissolved substances. That is why the term can describe anything from a rustic open-air bath by a mountain stream to a large resort spa. The labeling matters, because it tells guests that the water has been tested and described.

Most baths show a recognizable sign. Look for the hot-spring mark (a bowl with three wavy steam lines), the kanji 湯 (yu, "hot water"), or the hiragana ゆ. These appear on lanterns, curtains and station maps all over the country. Whenever you see them, you have found a place to soak, wash and unwind.

Inside many facilities, an analysis sheet describes the water in detail. The key line is the spring quality, written as sensitsu (泉質). It tells you the main type of water you are about to enjoy. Reading that single line can turn an ordinary bath into a small discovery, because you learn why the water looks, smells and feels the way it does.

The Main Water Types

The Japan Onsen Association describes about ten main spring qualities, which are determined by the chemical compounds in the water and their amounts. The gentlest is the simple spring, with dissolved ingredients of 1,000 milligrams per kilogram or less and a source temperature of at least 25 degrees Celsius. It is described as mild, with little skin irritation, and a simple spring with a higher pH is called a simple alkaline spring, which often feels smooth in the bath. Because it is so gentle, simple spring water is a comfortable first choice for newcomers.

Chloride springs have chloride as the main ingredient and taste salty. Hydrogen carbonate springs are built around hydrogen carbonate ions, and some types leave mineral deposits around the edges of the bath. Sulfate springs have sulfate as the main component. All three have at least 1,000 milligrams of dissolved ingredients per kilogram, so they are richer in minerals than simple springs.

Then there are the more distinctive types. Carbon dioxide springs are known for small bubbles that cling to your skin, which feels light and refreshing, and they are relatively rare. Iron springs contain at least 20 milligrams of iron per kilogram, and the water turns reddish as the iron meets the air. Acidic springs taste sour and have a distinctly lively feel. Iodine springs can turn yellow when left standing, and sulfur springs carry the unmistakable scent of hydrogen sulfide, often compared to boiled eggs. A final category, radioactive springs, is defined by radon content.

Colors, Smells and Famous Examples

One of the pleasures of onsen hopping is how different the baths look. Sulfur springs are often cloudy white or pale grey-green, while iron-rich baths tend toward a warm rust or golden tone as the water oxidizes. Simple and chloride springs are frequently clear. Some famous baths glow in unusual shades because of the way minerals interact with air and light, which makes them popular with photographers.

Several towns are well known for particular water. Kusatsu in Gunma is famous for its strongly acidic water and its steaming central hot-water field, and Arima near Kobe is known for its reddish "gold" spring rich in iron and salt. Beppu in Kyushu is celebrated for the sheer variety of its springs, with many different types in one city. Visiting a town known for one style is a nice way to learn what that style feels like, and the towns themselves are often charming places to wander in yukata and sandals.

Scent is part of the fun too. A sulfur bath's smell is a sign of its character, not a sign that anything is wrong, and many fans find it a comforting hallmark of a good hot spring. If you are sensitive to strong smells, choose a simple or chloride spring, and if you enjoy atmosphere, a sulfur bath will give you plenty of it.

Etiquette and Care

Good onsen etiquette is simple and makes every bath more pleasant. Wash and rinse thoroughly at the washing stations before entering the water, keep long hair tied up and out of the bath, and keep your small towel out of the water by setting it aside or folding it on your head. Traditional baths are enjoyed without swimsuits and are usually separated by gender, although private family baths exist at some inns. Many bathers skip a second shower so that the minerals stay on their skin, and it is polite to dry off a bit before returning to the changing area.

A few water-specific tips help too. Acidic springs can sting if you have cuts or fresh shaving nicks, so it is wise to be careful about them. Sulfur water can tarnish silver jewelry, and iron-rich water may stain pale towels or swimwear, so take off rings and bring a darker towel if you can. Take breaks, drink water, and step out when you feel warm, since good soaking is about comfort, not endurance.

Tattoos deserve a short note. Policies vary widely. Surveys reported by reference works found that many operators have restricted tattooed guests, while a good number allow them, sometimes with a cover patch, and some towns are known for being welcoming. Check the facility's policy before you go, or consider a private bath, and you will avoid any surprises.

Quick Takeaways

Onsen water types come down to which minerals dominate: simple, chloride, hydrogen carbonate, sulfate, carbon dioxide, iron, acidic, iodine, sulfur or radon. Look for the hot-spring mark and the sensitsu line on the analysis sheet to learn what you are bathing in. Start with a simple spring if you are new, and add a sulfur, iron or acidic bath when you want something with more character.

Wash first, keep your towel out of the water, and take care with jewelry and fresh cuts. With those few habits in mind, every bath becomes a small adventure, and the differences from town to town turn a simple soak into a tour of Japan's remarkable natural variety.

Frequently Asked Questions

What are the main types of onsen water?

The Japan Onsen Association describes about ten spring qualities, including simple, chloride, hydrogen carbonate, sulfate, carbon dioxide, iron, acidic, iodine, sulfur and radioactive springs. Each is defined by its main dissolved ingredients. That makes the list easy to follow.

Why do some onsen smell like eggs?

Sulfur springs contain hydrogen sulfide, which produces a smell often compared to boiled eggs. The scent is a normal feature of that spring type, and many visitors treat it as a sign of an authentic sulfur bath.

Which onsen type is best for beginners?

Simple springs are often recommended because they are described as mild with little skin irritation. They have 1,000 milligrams per kilogram or less of dissolved ingredients, so they feel gentle for first-time bathers. Many travelers find them a comfortable starting point.

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