We use broadband seismic data acquired by the St. Elias Erosion/Tectonics Project (STEEP) and the Alaska Earthquake Information Center to image the geometry of the subducting Yakutat Block in southeast Alaska. We combine results for both P- and S-wave receiver functions. P-wave to S-wave data were imaged with a fully three-dimensional wavefield imaging method centered on the STEEP region. The S-wave to P-wave data were imaged using a simpler common conversion point stacking method at two scales: a regional scale covering all of southeast Alaska and a smaller scale identical to the P-wave data. Our data confirm that the southeastern Alaska subduction zone extends from the eastern end of the Aleutian Trench an additional 300 km to the Fairweather–Queen Charlotte fault system. We also locate the boundary between the Yakutat Block and North American Plate. We find direct evidence that the subducted Yakutat Block and Pacific plate slabs are continuous and that Yakutat Block subduction extends from Prince William Sound to the east at least as far as Icy Bay. The dip angle of the slab ranges from 11° to 16° with a gradual increase from west to east across this region. Our data show a clear separation between the subducted Yakutat Block and the North American Plate under the Alaska Range, suggesting that deformation along the Denali fault and interior Alaska is not the product of coupling between North America and the subducted Yakutat Block. This dip angle also places the subducted Yakutat Block at the proper depth to produce arc magmatism found in the Wrangell volcanic field. Modeling the geometry of the system suggests that sedimentary cover is being stripped at the western side of the Yakutat Block and the lower crust of the Yakutat Block is involved in the subduction. The system transitions from a single dipping megathrust on the western side of the Yakutat Block to intense shortening in the vicinity of Mount Saint Elias, where we suggest that the lower crust is likely undergoing ductile deformation and has thickened more than 60 km under the Peninsular terrane.