We estimate the one- and three-dimensional P wave velocity structure beneath the Toba caldera complex, a 30 × 100 km topographic depression in North Sumatra, using arrival time data of local earthquakes recorded by a 40-station seismic network that operated for 4 months. Inversions reveal the presence of P velocities up to 37% below normal that likely map the distribution of magma within this subduction-related volcanic system, considered the world's largest. In the upper 10 km of crust the largest low-velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome. A smaller volume of low velocities is observed in the upper crust under the north end of the depression. Separating the two regions is a zone of locally high velocities, indicating that the shallow, subcaldera magma system is composed of two separate reservoirs, not a single one that extends the entire length of the caldera complex. Above each low-velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse ∼74 kyr ago. A low-velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low-frequency earthquakes in the 20- to 40-km-depth range. The low-frequency earthquakes apparently record the migration of melt in the mafic roots of this large-volume, crustal magma system.