Matsuogawa Onsen is a hidden gem nestled along the Matsuogawa Valley in Ikeda-cho, Miyoshi City, Tokushima Prefecture. With a pH of 10.2, it boasts one of the highest alkalinity levels of any hot spring in Japan. Located in the mountains near the famous Iya Valley and Oboke Gorge, this secluded spring has long been cherished by locals as a beautifying bath.

The water is classified as an alkaline simple sulfur spring (low mineral content, alkaline, low temperature). The source temperature is 25.0 degrees Celsius, classified as a cold mineral spring, and is heated using firewood before being delivered to the baths. The spring operates on a completely natural free-flowing system with no dilution or chemical disinfection whatsoever.

The water has a distinctive silky, slippery texture that clings to the skin, accompanied by a subtle sulfur aroma. The highly alkaline pH of 10.2 gently dissolves dead skin cells, leaving bathers with remarkably smooth skin — earning the spring its reputation as one of Shikoku''s finest beautifying baths. The indoor stone baths are separated by gender and offer views of the lush Matsuogawa Valley and its crystal-clear stream. The adjacent self-catering lodge Shirasagi-so provides affordable accommodation for extended therapeutic stays starting at 4,500 yen per night. Day-trip bathing costs 580 yen for adults and is available from 10:00 to 19:00, closed on Wednesdays.