What is onsen? Learn the Onsen Law definition, the link between onsen, mineral springs, and therapeutic springs, how they differ from heated bathwater, and why Japan has so many, based on Ministry of the Environment standards.
Published: Oct 22, 2025
What is onsen? Learn the Onsen Law definition, the link between onsen, mineral springs, and therapeutic springs, how they differ from heated bathwater, and why Japan has so many, based on Ministry of the Environment standards.
Published: Oct 22, 2025
Onsen refers to hot water or gas that springs from underground and meets the conditions set by Japan's Onsen Law. It does not simply mean a place with hot water; even if the temperature is low, it is treated as onsen as long as either the source temperature or the contained components meet the standard.
In other words, onsen is a legal term, not just a matter of feeling. This article organizes only the basic points you need first: the definition, the relationship between onsen, mineral springs, and therapeutic springs, the difference from ordinary heated bathwater, and why Japan has so many onsen. More detailed topics, such as spring water types and how hot springs emerge, are covered in separate articles.
In Japan, the Onsen Law defines onsen as hot water, mineral water, steam, or other gas that springs from underground and either has a source temperature of 25°C or higher or contains at least one of 19 designated components in the prescribed amount. The Ministry of the Environment oversees this standard.
What matters here is that you do not need to satisfy both the temperature and component conditions; one is enough. So even a cold spring below 25°C can legally count as onsen if it contains the specified components. Conversely, even if the special components are weak, a source at 25°C or higher is recognized as onsen. It is not unusual for onsen to look completely colorless and transparent.
| Condition | Details | Result |
|---|---|---|
| Temperature | Source temperature of 25°C or higher | Onsen if met |
| Components | One of 19 designated components at or above the required amount | Onsen if met |
The 19 items include total dissolved solids, free carbon dioxide, lithium ions, hydrogen ions, iodide ions, total sulfur, radon, and others. These are the names of components, not the spring water type names discussed later.
In the onsen world, the terms "mineral spring" and "therapeutic spring" also appear, which can be confusing. They are not separate categories; it is easier to understand them as having different levels of scope.
| Term | General meaning | Scope |
|---|---|---|
| Mineral spring | A general term for underground water with distinctive temperature or components, including cold mineral springs | Broadest |
| Onsen | Mineral springs that meet the temperature or component conditions of the Onsen Law | Middle |
| Therapeutic spring | Onsen that meet the Ministry of the Environment standards and can be used for treatment purposes | Narrowest |
A therapeutic spring is a subset of onsen and refers to water that meets stricter component standards. For example, total sulfur only needs to be 1 mg or more for onsen, but therapeutic springs require 2 mg or more. Only therapeutic springs are labeled with spring water types such as sulfur spring or bicarbonate spring, which you may see in changing rooms. The different spring water types and how to identify them are covered in the Guide to Onsen Spring Types.
The difference between onsen and a home bath or a large bath in a business hotel lies in the source of the water. Heated bathwater is simply tap water or similar water that has been heated, and it has no temperature or component standards like onsen. Onsen, on the other hand, originates from water that naturally springs from underground.
That said, onsen does not always mean completely untouched by human adjustment. In actual bath areas, water may be diluted to cool overly hot water, heated to warm lukewarm water, or circulated and filtered for hygiene. Since these practices are often displayed together with the onsen analysis report, you can tell how the water is being used by checking the sign. For a broader classification of facilities themselves, such as how onsen differ from sento and super sento, see The Difference Between Onsen and Sento.
Japan has so many onsen largely because it is one of the most volcanic countries in the world. Thanks to abundant geothermal heat underground, groundwater is heated and springs up in many places. Even in areas far from volcanoes, the deeper underground becomes warmer, so drilling deep enough can sometimes produce heated water.
The mechanism by which water is heated, viewed from the perspective of water, heat, and pathways, is explained in How Onsen Springs Up. For a broader look at why Japanese onsen are special, including the number of springs, the variety of spring types, and ryokan and hot spring town culture, see Why Japanese Onsen Are Special. This article is simply the entry point to what onsen are.
You do not need to memorize every definition to enjoy onsen. The three things worth understanding first are these.
First, onsen is a legally defined term, and water qualifies as onsen if it meets either the temperature or the component requirement. Second, spring water type names are used only for therapeutic springs, and onsen can be colorless and transparent. Third, even for the same onsen, the experience changes depending on whether the water is diluted, heated, or circulated, so it helps to get into the habit of checking the facility's signage.
Once you understand that, you can move on to articles about spring types and how hot springs emerge according to your interests. Bathing etiquette before actually entering is summarized in How to Use Onsen and Etiquette.
Yes. Under the Onsen Law, even if the source temperature is below 25°C, it is considered onsen as long as one of the 19 designated components is present in the prescribed amount. Either the temperature or the components need to meet the requirement.
Mineral spring is the broadest general term. Onsen are those that meet the conditions of the Onsen Law, and therapeutic springs are those that meet the stricter Ministry of the Environment standards and can be used for treatment purposes. Therapeutic springs are part of onsen.
Yes. Color and smell depend on the spring water type and are not conditions for being onsen. If the source temperature is 25°C or higher, it qualifies as onsen even if it looks just like plain water.
Those that meet the standards for therapeutic springs. There are 10 spring water types, and the differences between them are explained in Guide to Onsen Spring Types.
Onsen refers to hot water or gas that springs from underground and meets either a source temperature of 25°C or higher or one of the 19 designated components under the Onsen Law. Only one of the two conditions is needed, so onsen can be cold or colorless. Mineral springs, onsen, and therapeutic springs differ in scope, and only the narrowest category, therapeutic springs, receives spring water type names. Start by understanding this definition and relationship, then move on to separate articles about spring types, how onsen emerge, and facility differences for deeper understanding.
Onsen refers to hot water or gas that springs from underground and meets the conditions set by Japan's Onsen Law. It does not simply mean a place with hot water; even if the temperature is low, it is treated as onsen as long as either the source temperature or the contained components meet the standard.
In other words, onsen is a legal term, not just a matter of feeling. This article organizes only the basic points you need first: the definition, the relationship between onsen, mineral springs, and therapeutic springs, the difference from ordinary heated bathwater, and why Japan has so many onsen. More detailed topics, such as spring water types and how hot springs emerge, are covered in separate articles.
In Japan, the Onsen Law defines onsen as hot water, mineral water, steam, or other gas that springs from underground and either has a source temperature of 25°C or higher or contains at least one of 19 designated components in the prescribed amount. The Ministry of the Environment oversees this standard.
What matters here is that you do not need to satisfy both the temperature and component conditions; one is enough. So even a cold spring below 25°C can legally count as onsen if it contains the specified components. Conversely, even if the special components are weak, a source at 25°C or higher is recognized as onsen. It is not unusual for onsen to look completely colorless and transparent.
| Condition | Details | Result |
|---|---|---|
| Temperature | Source temperature of 25°C or higher | Onsen if met |
| Components | One of 19 designated components at or above the required amount | Onsen if met |
The 19 items include total dissolved solids, free carbon dioxide, lithium ions, hydrogen ions, iodide ions, total sulfur, radon, and others. These are the names of components, not the spring water type names discussed later.
In the onsen world, the terms "mineral spring" and "therapeutic spring" also appear, which can be confusing. They are not separate categories; it is easier to understand them as having different levels of scope.
| Term | General meaning | Scope |
|---|---|---|
| Mineral spring | A general term for underground water with distinctive temperature or components, including cold mineral springs | Broadest |
| Onsen | Mineral springs that meet the temperature or component conditions of the Onsen Law | Middle |
| Therapeutic spring | Onsen that meet the Ministry of the Environment standards and can be used for treatment purposes | Narrowest |
A therapeutic spring is a subset of onsen and refers to water that meets stricter component standards. For example, total sulfur only needs to be 1 mg or more for onsen, but therapeutic springs require 2 mg or more. Only therapeutic springs are labeled with spring water types such as sulfur spring or bicarbonate spring, which you may see in changing rooms. The different spring water types and how to identify them are covered in the Guide to Onsen Spring Types.
The difference between onsen and a home bath or a large bath in a business hotel lies in the source of the water. Heated bathwater is simply tap water or similar water that has been heated, and it has no temperature or component standards like onsen. Onsen, on the other hand, originates from water that naturally springs from underground.
That said, onsen does not always mean completely untouched by human adjustment. In actual bath areas, water may be diluted to cool overly hot water, heated to warm lukewarm water, or circulated and filtered for hygiene. Since these practices are often displayed together with the onsen analysis report, you can tell how the water is being used by checking the sign. For a broader classification of facilities themselves, such as how onsen differ from sento and super sento, see The Difference Between Onsen and Sento.
Japan has so many onsen largely because it is one of the most volcanic countries in the world. Thanks to abundant geothermal heat underground, groundwater is heated and springs up in many places. Even in areas far from volcanoes, the deeper underground becomes warmer, so drilling deep enough can sometimes produce heated water.
The mechanism by which water is heated, viewed from the perspective of water, heat, and pathways, is explained in How Onsen Springs Up. For a broader look at why Japanese onsen are special, including the number of springs, the variety of spring types, and ryokan and hot spring town culture, see Why Japanese Onsen Are Special. This article is simply the entry point to what onsen are.
You do not need to memorize every definition to enjoy onsen. The three things worth understanding first are these.
First, onsen is a legally defined term, and water qualifies as onsen if it meets either the temperature or the component requirement. Second, spring water type names are used only for therapeutic springs, and onsen can be colorless and transparent. Third, even for the same onsen, the experience changes depending on whether the water is diluted, heated, or circulated, so it helps to get into the habit of checking the facility's signage.
Once you understand that, you can move on to articles about spring types and how hot springs emerge according to your interests. Bathing etiquette before actually entering is summarized in How to Use Onsen and Etiquette.
Yes. Under the Onsen Law, even if the source temperature is below 25°C, it is considered onsen as long as one of the 19 designated components is present in the prescribed amount. Either the temperature or the components need to meet the requirement.
Mineral spring is the broadest general term. Onsen are those that meet the conditions of the Onsen Law, and therapeutic springs are those that meet the stricter Ministry of the Environment standards and can be used for treatment purposes. Therapeutic springs are part of onsen.
Yes. Color and smell depend on the spring water type and are not conditions for being onsen. If the source temperature is 25°C or higher, it qualifies as onsen even if it looks just like plain water.
Those that meet the standards for therapeutic springs. There are 10 spring water types, and the differences between them are explained in Guide to Onsen Spring Types.
Onsen refers to hot water or gas that springs from underground and meets either a source temperature of 25°C or higher or one of the 19 designated components under the Onsen Law. Only one of the two conditions is needed, so onsen can be cold or colorless. Mineral springs, onsen, and therapeutic springs differ in scope, and only the narrowest category, therapeutic springs, receives spring water type names. Start by understanding this definition and relationship, then move on to separate articles about spring types, how onsen emerge, and facility differences for deeper understanding.