Seamount subduction along subduction-zone plate boundary thrusts has long been implicated as a mechanism for abrasion and tectonic erosion of the base of the overriding plate. However, tectonic erosion processes have not been examined in detail with high-quality three-dimensional (3-D) seismic reflection imaging. In 1999 we acquired 3-D seismic reflection data from the Nankai Trough subduction zone to image the plate boundary fault and the overlying accretionary wedge structure. Fortuitously, these data reveal a small (to 1 km high) basement ridge that has subducted to 7 km subseafloor. Updip from the basement ridge, a 1-km-thick sequence of sediment from the base of the accretionary wedge appears to be missing. We interpret these data as evidence for tectonic erosion of the base of the accretionary wedge following the basement ridge subduction. Tectonic erosion has removed more than 25 km3 from the updip edge of the seismogenic zone and carried it down into the seismogenic zone. The tectonically eroded sediments are presumed to enhance fault-zone fluid content, potentially reducing fault-zone effective stress, and may temporarily inhibit earthquake rupture potential. After the passage of the ridge the boundary fault returns to its former position in a period of enhanced underplating.