P‐to‐S‐converted waves observed in controlled‐source multicomponent ocean bottom seismometer (OBS) records were used to derive the Vp/Vsstructure of Cascadia Basin sediments. We used P‐to‐Swaves converted at the basement to derive an empirical function describing the average Vp/Vsof Cascadia sediments as a function of sediment thickness. We derived one‐dimensional interval Vp/Vsfunctions from semblance velocity analysis of S‐converted intrasediment and basement reflections, which we used to define an empirical Vp/Vsversus burial depth compaction trend. We find that seaward from the Cascadia deformation front, Vp/Vsstructure offshore northern Oregon and Washington shows little variability along strike, while the structure of incoming sediments offshore central Oregon is more heterogeneous and includes intermediate‐to‐deep sediment layers of anomalously elevated Vp/Vs. These zones with elevated Vp/Vsare likely due to elevated pore fluid pressures, although layers of high sand content intercalated within a more clayey sedimentary sequence, and/or a higher content of coarser‐grained clay minerals relative to finer‐grained smectite could be contributing factors. We find that the proto‐décollement offshore central Oregon develops within the incoming sediments at a low‐permeability boundary that traps fluids in a stratigraphic level where fluid overpressure exceeds 50% of the differential pressure between the hydrostatic pressure and the lithostatic pressure. Incoming sediments with the highest estimated fluid overpressures occur offshore central Oregon where deformation of the accretionary prism is seaward vergent. Conversely, landward vergence offshore northern Oregon and Washington correlates with more moderate pore pressures and laterally homogeneous Vp/Vsfunctions of Cascadia Basin sediments. Average and interval Vp/Vsversus depth functions derived from S‐converted waves from the Ridge‐to‐Trench seismic experiment in Cascadia BasinLayers with high Vp/Vsindicate elevated pore fluid pressure in incoming sedimentsVariations in Vp/Vsstructure correlate with underthrust sediment thickness and wedge deformation style