Hyakkannon Onsen is a remarkably rare self-spouting natural hot spring located on the Kanto Plain in Nishi-Owa, Kuki City, Saitama Prefecture. Its name derives from the Edo period, when the estate''s owner made pilgrimages to all 100 Kannon (Goddess of Mercy) temples across the Bando 33, Saigoku 33, and Chichibu 34 sacred sites, collecting talismans and sand from each to build a Kannon hall housing 100 statues on the property. Inspired by this spiritual heritage, deep boring to 1,500 meters was undertaken, and the facility opened as a day-trip bathing establishment in February 2002.

The spring water is classified as a sodium chloride strong salt spring with a source temperature of 57 degrees Celsius and an abundant flow rate of 1,000 liters per minute. With a pH of 7.9 (mildly alkaline) and a total mineral content of 18,510 mg/kg, the water is exceptionally concentrated. Sourced from ancient fossil seawater and self-spouting alongside natural gas, the facility achieves 100% free-flowing source water with no dilution or heating. The Japan Spa Association awarded perfect 5-star ratings across all six evaluation categories — a distinction held by fewer than 20 facilities nationwide.

The facility features outdoor rock baths, outdoor carbonated baths, indoor baths, reclining baths, waterfall baths, cold water baths, sauna, and five private bath rooms. The on-site restaurant Shochikutei serves meals. Admission is 850 yen for adults on weekdays and 900 yen on weekends and holidays. Hours are 8:00-23:00 on weekdays and 7:00-23:00 on weekends. The facility is conveniently located just a 3-minute walk from JR Higashi-Washinomiya Station on the Utsunomiya Line.