The rules aren’t complicated, but they’re rarely explained to visitors. Here’s what matters.
Wash before you soak
The bath is for soaking, not cleaning. Shower stations with soap and shampoo sit before the tub — rinse off completely, hair included, before getting in. Skipping this is the #1 mistake foreign visitors make.
Naked, gender-separated
No swimsuits. Entrances split by gender (red/pink = women, blue = men, or 女/男). Everyone bathes nude — normalized and unremarkable to regulars.
The small towel stays out of the water
Use it to wash. Fold it on your head or beside the tub while soaking — never in the bathwater.
Tattoos: check first
Many onsen still refuse visibly tattooed guests (a holdover tied to organized crime, not tattoos themselves). Some now allow cover stickers or private baths. Check the specific facility, or book a ryokan with a private in-room bath.
Long hair goes up
Hair touching the water is considered unclean for the next bather.
Take it slow
Water runs 40-44°C (104-111°F) — hotter than home baths. Ease in, exit if lightheaded, and skip it entirely after heavy drinking.
Why bother
Get this right and an onsen is one of the best parts of a Japan trip — especially a ryokan rotenburo (outdoor bath) after a day of hiking or a long Shinkansen ride.