Onsen (natural hot spring baths) and sentō (public bathhouses, not necessarily fed by a natural spring) follow the same basic etiquette, and getting it right isn't complicated — it just isn't always intuitive if you've never done it before.
Wash thoroughly before entering
The bath itself is for soaking, not washing — nearly every onsen and sentō provides a row of individual washing stations, with stools, handheld showers, soap and shampoo, where you wash and rinse completely before ever entering the shared water. Skipping this step is the single most common first-timer mistake.
No swimwear
Bathing is done nude; swimwear isn't worn in the water at a traditional onsen or sentō. Baths are separated by gender (occasionally with mixed options at some rural or historic onsen, always clearly signed). This is standard practice, not exhibitionism — everyone follows the same rule, and no one pays it any attention.
The small towel
You'll typically be given or can rent a small modesty towel — use it to cover yourself while walking around, but keep it out of the bath water itself; most people leave it folded on their head or set aside at the water's edge while soaking.
Tattoos
Many onsen and sentō in Japan still restrict visible tattoos, a longstanding policy tied to their historic association with organised crime, though this is loosening at some establishments, particularly ones used to international visitors. If you have visible tattoos, it's worth checking a specific venue's policy ahead, looking for tattoo-friendly listings, or using waterproof cover patches, which some places accept.