Mount Iō (硫黄岳 Iō-dake?) is a mountain on the border of Chino and Minamimaki, Nagano Prefecture, Japan. This mountain is one of the major mountains of Yatsugatake Mountains. Mount Iō literally means, sulphur mountain. Mount Iō is a stratovolcano in Southern Yatsugatake Volcanic Group. This mountain also belongs to the Yatsugatake-Chūshin Kōgen Quasi-National Park.
This mountain is one of the major mountains in Yatsugatake Mountains, which is easy for climbers to reach to the top. The easiest route is from Sakurazawa.