Hannyaji Onsen is a secluded hot spring located along a tributary of the Yoshii River in the Okutsu Onsen area of Kagamino Town, Tomata District, Okayama Prefecture. Originally established in 1871 as a lodging for Hannyaji Temple of the Tendai Buddhist sect, it began operating as a hot spring inn in 1957. The facility is renowned for its open-air bath offering close-up views of Ayugaeshi Falls, one of the Eight Scenic Spots of Okutsu, accessed via a path through bamboo groves.

The spring water is classified as alkaline simple hot spring with a remarkably high pH of 9.6, earning it the reputation of a bijin-no-yu (beauty spring). The property has three private source springs, with the indoor bath fed by a blend of naturally flowing 39 and 40 degree Celsius sources, while the outdoor bath uses 40 to 42 degree Celsius spring water. The water is delivered as 100 percent free-flowing source water, characterized by its silky-smooth texture and crystal clarity.

The facility operates on a one-hour private reservation system, allowing guests exclusive use of both indoor and outdoor baths. The rustic compound consists of a thatched-roof main building and three cottage rooms, where bathers can soak while surrounded by the sounds of the river and waterfall. Admission is 1,100 yen for adults and 550 yen for children, with an additional 1,100 yen private rental fee. The outdoor bath operates seasonally from mid-April to early December.