Toyotomi Onsen is the northernmost hot spring resort in Japan, located in Toyotomi Town, Teshio District, Hokkaido. Discovered in 1926 during oil drilling when hot water gushed out with natural gas, it features a globally rare oil-containing fossil seawater spring. The weakly alkaline, hypertonic hot spring is gentle on the skin, with sodium bicarbonate and boron providing skin-beautifying and antiseptic effects. The tar components in the oil improve skin barrier function. Located near the Sarobetsu Plain with views of Mt. Rishiri, it is particularly effective for skin conditions and attracts visitors from across Japan seeking treatment for atopic dermatitis.