Avachinsky (also known as Avacha or Avacha Volcano or Avachinskaya Sopka) (Russian: Авачинская сопка, Авача) is an active stratovolcano on the Kamchatka Peninsula in the far east of Russia. It lies within sight of the capital of Kamchatka Krai, Petropavlovsk-Kamchatsky. Together with neighboring Koryaksky volcano, it has been designated a Decade Volcano, worthy of particular study in light of its history of explosive eruptions and proximity to populated areas.

Avachinsky's last eruption occurred in 2001. This eruption was tiny compared to the volcano's major Volcanic Explosivity Index 4 eruption in 1945.

Avachinsky lies on the Pacific Ring of Fire, at a point where the Pacific Plate is sliding underneath the Eurasian Plate at a rate of about 80 mm/year. A wedge of mantle material lying between the subducting Pacific Plate and the overlying Eurasian Plate is the source of dynamic volcanism over the whole Kamchatka Peninsula.

The volcano is one of the most active volcanoes on the Kamchatka Peninsula, and began erupting in the middle to late Pleistocene era. It has a horseshoe-shaped caldera, which formed 30-40,000 years ago in a major landslide which covered an area of 500 km² south of the volcano, underlying the city of Petropavlovsk-Kamchatsky. Reconstruction of a new cone inside the caldera occurred in two major eruption phases, 18,000 and 7,000 years ago.