Ubayu Onsen is a secluded hot spring located at an elevation of 1,300 meters on the northern side of the Azuma mountain range in Yonezawa City, Yamagata Prefecture. It is the highest-altitude hot spring in Yamagata Prefecture, discovered in 1533 (Tenbun 2) during the Muromachi period. Counted as one of the "Yonezawa Eight Hot Springs," only one ryokan called "Masugataya" exists in this remote mountain location.

According to legend, the hot spring was discovered when the inn's ancestor, Endo Daikura, a gold prospector, got lost in the mountains and encountered an old woman (mountain witch) carrying a child. She showed him where the hot spring waters emerged and the way back home, leading to the name "Ubayu" (Old Woman's Hot Spring). The current owner is the 17th generation, Endo Hitoshi, and the family has been protecting this hot spring for over 490 years.

Located at the foot of Mount Dainichi (1,621m) in an ancient volcanic crater site, the mountain lodge-style building sits on the left bank of the Maegawa River, which flows from Mount Eboshiyama in the Azuma mountain range. Behind the inn towers Mount Yakushi (1,537m), while the strange rock formations of Mount Dainichi rise majestically across the river to the south, creating a truly secluded location surrounded by cliffs.

The hot spring source naturally emerges from the middle slopes of Mount Dainichi at an elevation of 1,250m, with a temperature of 51.1°C and an abundant flow rate of 400 liters per minute. The water quality is simple acidic sulfur spring with a pH of 2.5. The water is drawn 200 meters through PVC pipes from the source, temperature-adjusted through channels that collect sulfur flowers, and supplied as 100% source spring flowing water.

Particularly famous is the mixed outdoor bath called "Yamamba-no-yu" (Mountain Witch Bath), surrounded on three sides by towering cliffs of strange rocks and stones, offering an overwhelming landscape experience that cannot be found elsewhere. Visitors can enjoy the ultimate secluded hot spring experience while feeling the great nature throughout the four seasons - fresh greenery in spring, cool relief in summer, autumn foliage, and starry skies at night - making it beloved by hot spring enthusiasts.