The South Pole‐Aitken basin is a gigantic impact structure on the far side of the Moon, with an inner rim extending approximately 2,000 km in the long axis dimension. The structure and history of this basin are illuminated by gravity and topography data, which constrain the subsurface distribution of mass. These data point to the existence of a large excess of mass in the Moon's mantle under the South Pole‐Aitken basin. This anomaly has a minimum mass of 2.18 × 1018 kg and likely extends to depths of more than 300 km. Plausible sources for this anomaly include metal from the core of a differentiated impactor or oxides from the last stage of magma ocean crystallization. Although the basin‐forming impact event likely excavated the vast majority of the preexisting crust, the present‐day crust of the basin interior is at least 16 km thick in undisturbed regions.