Active‐source seismic surveys have resolved the fine‐scale P‐wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S‐wave velocity (Vs) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high‐frequency teleseismic waveforms. As a result, a thin (∼1 km) low‐velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip. Plain Language Summary Many geophysical surveys have investigated the subsurface structures of shallow subduction zones by estimating the propagation speed of compressional waves emitted from artificial explosive sources. Although shear and compressional wave speeds are necessary to understand rock properties (e.g., water pressure) of the subsurface, it has been difficult to constrain the shear wave speed with a high‐resolution. In this study, we estimate both compressional and shear wave speeds by applying an advanced technique to earthquake waveforms recorded at ocean‐bottom seismometers deployed at the Nankai subduction zone, Japan. The results show a sufficiently high spatial resolution to detect a thin (~1 km) layer, which we interpret as water‐rich subducted sediment. This water‐rich zone may promote slow slips on the megathrust fault and work as a barrier against rupture propagations during large earthquakes. Key Points We applied transdimensional inversion of teleseismic waveforms to seafloor cabled stations at the Nankai subduction zone The resultant high‐resolution velocity structures allow a direct comparison to active source surveys A low‐velocity zone beneath the outer ridge is evident, which is interpreted as an overpressured portion of underthrust sediment