Onsen-ji, Buddhist temple in Nikko Yumoto Onsen, Japan.

Onsen-ji is a Buddhist temple with an integrated public bath facility housed within the main complex. Visitors can soak in wooden tubs filled with naturally heated spring water, combining religious visits with traditional Japanese bathing.

Onsen-ji was founded in 788 by Shōdō, the monk credited with opening Rinnō-ji, and it serves as a branch temple (betsuin) of Rinnō-ji. Its mountain name is Nikkō-san and its principal image is Yakushi Nyorai, the Buddha of healing. The sources record no later rebuilding or storm damage.

The temple belongs to the Tendai school and is a branch temple (betsuin) of Rinnō-ji, with the mountain name Nikkō-zan and Yakushi Nyorai as its principal image. In the kuri, the priests' quarters, a bath called Yakusui-yu is fed directly from the spring and is open to anyone from mid-April to late November.

The temple sits in the highland area of Nikko and is accessible by bus from town, with a journey of around 80 minutes from the main station. Visitors should bring their own towels and wear layers, as the location can be quite cool even when the bath itself is warm.

The bath in the temple's kuri is fed directly from the source and is open to anyone from mid-April to late November. The spring water is a sulfurous calcium-sodium sulfate and hydrogen carbonate type at 71.4°C.