Tokigawa Onsen is a hidden hot spring nestled in the mountains of Tokigawa Town, Hiki District, Saitama Prefecture, discovered in 1990 when a construction company executive accidentally struck a hot spring while drilling a well for a vacation home. The spring boasts a pH of 11.3, making it the most alkaline hot spring in Japan — so remarkable that it is featured in high school chemistry textbooks nationwide. After a temporary closure following the bursting of the economic bubble, the current owner Mr. Nakano acquired the property and reopened it as a day-trip-only hot spring in 2014.

The water is classified as a strongly alkaline cold mineral spring qualifying as an onsen under the Hot Spring Law due to its metasilicic acid content. The source temperature is 14.9 degrees Celsius, with metasilicic acid exceeding 54.1mg — a natural moisturizing component that helps restore skin ceramides. The water is delivered as 100% pure source water with no dilution, circulation, or disinfection; only heating is applied. Each evening the bath is drained and cleaned, with the naturally flowing spring at 2.4 liters per minute refilling from morning to provide perfectly fresh water.

Limited to just 4 groups per day by reservation only, the facility operates in two sessions: a lunch session from 11:00 to 15:00 and an evening session from 16:00 to 20:00, accommodating 2 groups each. Guests enjoy a private bath crafted from cypress and natural stone, along with 4 hours of private room use and a seasonal kaiseki meal featuring locally grown yuzu citrus. Towels, yukata robes, and amenities are all provided for a hassle-free visit. This is a day-trip-only establishment and does not offer overnight accommodation.