Mount Iō (硫黄岳, Iō-dake) is a mountain on the border of Chino and Minamimaki, Nagano Prefecture, Japan.

In 100 Famous Japanese Mountains, Kyūya Fukada mentions the loss of a younger climbing mate, Kyoichi Yoshimura, who fell to his death while descending the northern cliffs of Mt Iō.

The summitt is accessible year-round but winter speciality climbing gear is necessary from October through April.

One of the easiest routes is from Sakuradaira (櫻平), 1,894 m (6,214 ft), and takes approximately 6 hours.

From Sakuradaira, the climb is approximately 5 km, and passes Natsusawa Kosen (夏沢鉱泉, Natsuzawa kousen), 2,054 m (6,739 ft), a mountain hut with hot spring that is open year-round, O-ren Hut (オーレン小屋, O-ren goya), 2,316 m (7,598 ft), a seasonly-opened mountain hut, and Natsuzawa Pass (夏沢峠, Natsuzawa touge), 2,423 m (7,949 ft).