Klyuchevskoy volcano in Kamchatka (Russia) is unique in the island arc systems of Earth in having nearly continuous seismic activity beneath it at depths in excess of 20 km. Seismograms from these deep earthquakes carry an unmistakable signature of their tectonic nature. We use P-to-S (compressional to shear) converted teleseismic waves to constrain the depth of the crust-mantle transition beneath Klyuchevskoy at ∼25 km, and to delineate a deeper seismic boundary at ∼50 km. Earthquakes directly beneath Klyuchevskoy have hypocentral depths of 25–35 km. S-P delays in records of these earthquakes are always larger than delay times of P-to-S converted waves originating at the crust-mantle transition and traversing nearly identical paths. Thus, deep seismic activity under Klyuchevskoy is definitely beneath the crust-mantle transition. Compositions of the Klyuchevskoy parental melts (inferred from melt inclusions and the most primitive lava) interpreted using a barometer based on Si activity in melts saturated with orthopyroxene + olivine show that Klyuchevskoy parental melts form at pressures within the range of 13.9 (±2) kbar (at depths of 46 ± 7 km). Together, the estimates of melting depths, the locations of seismic velocity features, and the occurrence of tectonic earthquakes all point to the existence of a subcrustal volume beneath Klyuchevskoy volcano where processes of magma accumulation are vigorous enough to promote brittle failure in mantle rock.