Large caldera-forming eruptions (CFEs, VEI ≥ 6) are one of the greatest and most globally transformative natural hazards. Knowledge of their location, history, and the magmatic processes that drive caldera formation is critical for understanding Earth’s past climate and the hazards associated with future eruptive activity. The eastern Islands of Four Mountains (IFM) in Alaska’s central Aleutian arc comprise a ring of six closely-spaced stratovolcanoes (Cleveland, Carlisle, Uliaga, Kagamil, Tana and Herbert) and several subsidiary cones and fissures. Here we present multi-disciplinary evidence including bathymetry, gravity, geologic deposits, earthquake hypocenters, gas chemistry and locations of fumaroles, and structural trends and volcanic vent morphology, that suggest that volcanism in the IFM is influenced by a large, previously unrecognized caldera largely hidden by the ocean and recent geologic deposits. The identification of such a caldera in this sector of the Aleutian arc advances our understanding of the interplay between large eruptions and climate, the processes associated with the generation and accumulation of magma in the shallow crust, and the persistent long-term eruptive activity at Mount Cleveland (North America’s most active volcano) and at satellite vents in similarly-sized caldera systems worldwide.