In Japan, the etiquette regarding clothing in hot springs, known as onsen, varies depending on the specific onsen and its rules. Generally, in most traditional onsen in Japan, it is customary to bathe naked, without any clothing or swimwear. The idea is to keep the onsen water clean, and wearing swimwear is often seen as unhygienic. However, some modern onsen resorts may allow swimwear in certain areas or have mixed-gender baths where specific swimwear is required. It's essential to check the rules of the particular onsen you plan to visit to ensure you adhere to their guidelines.