The thrust mechanism of the 2012 Mw 7.8 Haida Gwaii earthquake suggests convergence across the transpressive Pacific–North America plate boundary in the region is accommodated by underthrusting, with important consequences for seismic‐ and tsunami‐hazard analysis. This article investigates the crustal structure and extent of subduction beneath Haida Gwaii by nonlinear inversion of receiver function data processed from teleseismic recordings at five land‐based seismograph stations. Three of these stations were deployed since the 2012 earthquake to extend coverage to the southeast and have not been analyzed previously. The inversions provide estimates of the shear‐wave velocity structure beneath much of Moresby Island. Results indicate a positive velocity contrast at approximately 18–26 km depth, interpreted as a shallow continental Moho. A 12–17 km thick low shear‐wave velocity zone is also identified, which increases in depth from ∼25 to 42 km along the direction of plate convergence, which is interpreted as subducting oceanic material. These results provide the first evidence that the subducting oceanic plate extends beneath the entirety of Moresby Island.