Reiseniji Onsen is a small, rustic hot spring village nestled in the mountains of Ueda City, Nagano Prefecture, consisting of only four ryokan (traditional Japanese inns). The hot spring's history dates back to the Heian period, with legends claiming that in 968 AD, when Kongozan Reiseniji Temple was founded, hot spring water began flowing from beside the temple. Another legend tells of Taira no Koremochi healing his wounds here after defeating the demon woman of Togakushi. The area was designated as a National Health Resort Hot Spring in 1956. The water is an alkaline simple hot spring (low osmotic pressure, alkaline, high temperature) with a pH of 8.9 and a source temperature of 43.8 degrees Celsius. The colorless and transparent water is renowned as a beauty spring that leaves skin silky smooth. The communal bathhouse offers 100% free-flowing natural hot spring water in a tranquil setting with no restaurants, souvenir shops, or convenience stores, preserving the authentic atmosphere of a traditional toji (hot spring cure) village. It is part of the Maruko Onsen District along with Kakeyu and Oshio hot springs, which have been beloved as therapeutic bathing destinations since the Edo period.