A complex volcano, also called a compound volcano, is a volcano with more than one feature. They form because changes of their eruptive characteristics or the location of multiple vents in an area. Stratovolcanoes may form complex volcanoes, because they may overlap another from repeated explosive eruptions, lava flows, pyroclastic flows to make multiple summits and vents. Stratovolcanoes can also form a large caldera that gets filled in by multiple small cinder cones, lava domes and craters may also develop on the caldera's rim.

Although a comparatively unusual type of volcano, they are widespread in the world and in geologic history. Metamorphosed ash flow tuffs are widespread in the Precambrian rocks of northern New Mexico, which describes that caldera complexes have been important for much of the Earth's history. Yellowstone National Park is on three partly covered caldera complexes. The Long Valley Caldera in eastern California is also a complex volcano; the San Juan Mountains in southwestern Colorado are formed on a group of Tertiary-age caldera complexes, and most of the Mesozoic and Tertiary rock of Nevada, Idaho, and eastern California are also caldera complexes and their erupted ash flow tuffs. The Bennett Lake Caldera in British Columbia and the Yukon Territory is another example of a Tertiary-age caldera complex.