This paper presents a three-dimensional compressional wave velocity model of the forearc crust and upper mantle and the subducting Juan de Fuca plate beneath southwestern British Columbia and the adjoining straits of Georgia and Juan de Fuca. The velocity model was constructed through joint tomographic inversion of 50,000 first-arrival times from earthquakes and active seismic sources. Wrangellia rocks of the accreted Paleozoic and Mesozoic island arc assemblage underlying southern Vancouver Island in the Cascadia forearc are imaged at some locations with higher than average lower crustal velocities of 6.5--7.2 km/s, similar to observations at other island arc terranes. The mafic Eocene Crescent terrane, thrust landward beneath southern Vancouver Island, exhibits crustal velocities in the range of 6.0--6.7 km/s and is inferred to extend to a depth of more than 20 km. The Cenozoic Olympic Subduction Complex, an accretionary prism thrust beneath the Crescent terrane in the Olympic Peninsula, is imaged as a low-velocity wedge to depths of at least 20 km. Three zones with velocities of 7.0--7.5 km/s, inferred to be mafic and/or ultramafic units, lie above the subducting Juan de Fuca plate at depths of 25--35 km. The forearc upper mantle wedge beneath southeastern Vancouver Island and the Strait of Georgia exhibits low velocities of 7.2--7.5 km/s, inferred to correspond to ~20% serpentinization of mantle peridotites, and consistent with similar observations in other warm subduction zones. Estimated dip of the Juan de Fuca plate beneath southern Vancouver Island is ~11¿, 16¿, and 27¿ at depths of 30, 40, and 50 km, respectively.