Mount Usu, Active volcano in Hokkaido, Japan.

Mount Usu is a 733-metre active volcano on the southern rim of the Toya Caldera, which holds Lake Tōya to the north. Its summit is a horseshoe-shaped outer rim about 1.8 km across, open to the south, enclosing lava domes such as Ōusu and Kousu. The flanks show pale grey and tan ash layers in diagonal bands, cut by erosion on the right side, with a small cluster of white structures among the trees high on the upper right.

The volcano erupted four times during the 20th century, producing Meiji-shinzan in 1910 and forming Shōwa-shinzan between 1944 and 1945. Additional eruptions followed in 1977 and most recently in 2000, when large sections of the surrounding area required evacuation.

The name Usu derives from the Ainu word us, meaning "bay", referring to the inlet on the coast of present-day Usu in Date. Japanese arrivals later likened the name to a mortar (usu) and wrote it as "Usu-dake". The mountain's eruptions also entered Ainu oral tradition, such as the tale of the village chief who stayed praying at the altar during the Bunsei eruption and was found burned to ash in that posture.

A ropeway runs from the base to viewing platforms overlooking Shōwa-shinzan, so the summit area is reached without a long hike. The 2000 eruption craters and the Usu Shinzan dome lie within walking distance of the upper station. Wind at the top station is colder than at the base.

During the 2000 eruption, over 16,000 people evacuated without any loss of life, an outcome internationally recognized as exemplary disaster preparedness. The warning systems and evacuation plans developed in the region have since served as a model for other volcanic areas worldwide.