What is the difference between an onsen and a sento

What legally makes water an onsen

In Japan, onsen is a legal term. Article 2 of the Hot Springs Act covers hot water, mineral water, steam and other gases that come out of the ground, as long as they meet a condition in the law's appended table. The table offers two routes. Either the water is 25 C or warmer when it is taken from the source, or it contains a set amount of at least one listed substance.

The Japan Onsen Association counts 19 substances on that list. They include 1,000 mg of dissolved solids per kilogram of water, 250 mg of free carbon dioxide, 1 mg of lithium ions and 50 mg of metasilicic acid, among others. Either route is enough on its own. A spring that comes out cold can still be an onsen because of what is dissolved in it, and a hot one qualifies on temperature even if it carries very little.

Cold onsen are not a curiosity. In its survey of hot spring use for fiscal 2024, Japan's Ministry of the Environment counted 27,899 hot spring sources, and 4,074 of them came out of the ground below 25 C. The same survey counted 2,839 hot spring areas, which the ministry only records where there is lodging, and 7,721 public baths that use hot spring water.

What a sento is

A sento is a public bathhouse in the plainest sense. As the Ministry of Health, Labour and Welfare summarizes the Public Bathhouse Act, a public bath is any facility that lets the public bathe using hot water, seawater, hot spring water or anything else, and each one needs a license from the prefecture. The law then splits public baths into two groups. General public baths serve the daily hygiene of local residents, and their admission is controlled under the Prices Control Ordinance. That group is what people mean by sento. Health centers, baths at sports facilities, saunas and private-room baths fall into the second group.

The water is usually nothing special. Stripes Japan notes that most sento use tap or well water. What makes a sento a sento is its role in the neighborhood and its capped price, not its source.

Each prefecture sets its own cap. In Tokyo, the Tokyo Sento Association announced that the adult cap rose by 50 yen to 600 yen on August 1, 2026, after the metropolitan government's public bath council reviewed costs, while children aged 6 to 11 still pay 200 yen and children under 6 pay 100 yen. SoraNews24 reported that the previous change, in August 2024, was a 30 yen rise, and that this year's review pointed to higher labor, electricity and gas costs.

There are far fewer sento than there used to be. The health ministry's sanitation administration report for fiscal 2024 counted 2,730 general public baths nationwide, down from 3,231 in fiscal 2020. The second category, which includes large leisure baths, stood at 20,938.

Where the two overlap

The line blurs because the two definitions measure different things. The Hot Springs Act is about the water. The Public Bathhouse Act is about the business. A single bath can meet both. The Ota City Tourism Association notes that Ota ward has more public baths than any other area of Tokyo, and that many of them offer kuroyu, a black hot spring water whose color is said to come from ancient plants broken down by microorganisms deep underground. Those are neighborhood public baths filled with spring water.

Hot spring towns have their own version of the neighborhood bath. The travel site japan-guide.com describes the soto-yu of Nozawa Onsen in Nagano as more than a dozen small public bathhouses that have been community owned and maintained since feudal times. They are free, with a small donation asked for upkeep, and they provide no amenities.

Large leisure baths add a third kind of place. Inside Kyoto describes super sento as big complexes with saunas, steam rooms, massage, restaurants and many kinds of tubs, and points out that one of them, Kyoto's Tenzan-no-Yu, draws some of its water from a deep well and calls itself an onsen. Nippon.com places super sento in the other public bath category and notes that, unlike ordinary sento, these facilities are free to set their own prices. A hotel's large shared bath, the daiyokujo, is only an onsen if its water comes from a hot spring, as Inside Kyoto puts it.

How to tell which one you are in

Look for the posted analysis. Under Article 18 of the Hot Springs Act, anyone who offers hot spring water for public bathing must post the water's composition, its contraindications and bathing precautions in the easiest place to see in the facility, based on an analysis by a registered laboratory. According to the Aichi Prefectural Institute of Public Health, that analysis has had to be redone at least every ten years since October 20, 2007.

A second notice tells you how the water is handled. Since May 24, 2005, the Ministry of the Environment has required hot spring facilities to post whether they add water, heat it, recirculate it, add bath additives or disinfect it, and why. A bath can be a legal onsen and still be heated and filtered. This notice is where you find out.

At a sento, the price is the clue. The cap only applies to general public baths, so a Tokyo bath charging an adult more than 600 yen for basic entry belongs to the other category, whatever its sign says. A sento that fills its tubs from a spring is also offering hot spring water for public bathing, so it has to post the same analysis as any onsen.

What stays the same at both

The routine does not change much. The Tokyo Sento Association's WELCOME! SENTO guide asks bathers to keep phones out of the bathing and changing rooms, wash well and rinse off all the foam before getting in, keep towels and long hair out of the tub, keep their voices down, not wash clothes in the bathing room, and wipe off with a small towel before going back to the lockers.

The etiquette list of Beppu's Onsendo, a hot spring stamp program run from the Beppu City Tourism Association, reads much the same and adds a few points for communal onsen. It names skipping the pre-bath rinse, running the cold tap freely, loud conversation and sitting on the edge of the tub as the most disliked habits. Before adding tap water to a hot bath, it asks you to say a word to the people in it, because the water dilutes the spring. It also asks you to treat a communal bath as if you were visiting someone else's bathroom.

Bring your own small towel either way. The japan-guide.com page on Nozawa notes that its free soto-yu provide no amenities, and the WELCOME! SENTO page warns that renting towels, soap or shampoo may cost extra at Tokyo sento.

Tattoo rules are set by each bath. In its first survey on the subject, in June 2015, the Japan Tourism Agency asked 3,768 hotels, ryokan and similar facilities whether they let tattooed guests bathe. Nikkei reported that of the 581 that answered, 56 percent refused, 31 percent did not, and 13 percent allowed it with conditions such as covering the tattoo with a sticker. In March 2016 the agency told the hotel, ryokan and onsen associations that refusing someone only because of a tattoo is not appropriate, and listed options such as cover stickers and private baths. For sento, LIVE JAPAN reported in January 2025 that the Tokyo Sento Association listed 54 bathhouses under its WELCOME! SENTO banner, each of them tattoo-friendly.

Common mix-ups

Assuming an onsen has to be hot. The Ministry of the Environment's count of 4,074 sources below 25 C shows how many qualify on what is dissolved in them rather than on heat. If a cool source is warmed before it reaches the tub, the handling notice will say so.

Calling every big bath an onsen. A hotel daiyokujo or a super sento may run on tap water. Inside Kyoto's rule of thumb is that the water must come out of the ground, and the posted analysis is the proof.

Expecting a sento to be the cheapest bath in town. The cap is a ceiling, not a floor, and some onsen cost less. Nozawa's soto-yu cost nothing but a donation.

Mixing them up in your own records. If you keep a log of the baths you visit, tag each one by what it is rather than by the word on its sign. Yulog, a stamp book app for onsen and sento regulars, lets you mark each visit as an onsen, a sento or a thermal bath, and its Rank tab keeps a count by type. The tag is your own call. The app does not check the water.

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