Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3--8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200--300 km. The plate dips at a moderate angle of 45¿ to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North cascade Range of Washington, the plate strikes to the northwest and has 6--8% faster velocities than the upper mantle to the west. South of 47¿N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3--4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123¿W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.