Can Muslims go to a Japanese onsen?

What an onsen is

An onsen is a natural hot spring bath, a big part of Japanese culture. Most public baths are split into men's and women's sections, and guests wash and then soak completely naked. Swimsuits and towels in the water are usually not allowed. So the issue isn't the hot water; it's being uncovered in front of other people.

The Islamic rule on nudity

Islam asks Muslims to cover their awrah even in front of people of the same gender. For men, that's at least from the navel to the knee; for women in front of other women, the area between the navel and the knees by the majority view, with many recommending more. The Prophet ﷺ said, "A man should not look at the awrah of another man, nor a woman at the awrah of another woman" (Sahih Muslim 338). The Quran also says:

Bathing naked in a shared pool, and seeing others naked, makes it very hard to keep this. It's the same issue as European nude saunas. See Can I still go to the sauna as a Muslim?

Halal ways to enjoy hot springs

• Private onsen (kashikiri-buro): many ryokan inns rent private baths by the hour, perfect for you alone or with your spouse.
• Rooms with their own open-air bath: pricier, but completely private.
• Foot baths (ashiyu): free or cheap public foot baths where you just soak your feet.
• Swimwear spas: some resorts and theme spas allow swimsuits in mixed areas; check that everyone else is covered and the atmosphere suits you.
• Off-peak visits: some travelers find a bath empty at quiet hours, but you can't control who walks in, so private baths are safer.

Japan as a Muslim traveler

Japan has become much easier for Muslim visitors, with prayer rooms at major airports, halal ramen shops in big cities and clear food labels. See Which countries are Muslim-friendly for travel? Hot springs are part of what makes it special, and with a private bath you lose nothing of the experience.