The up-dip extent of slip during large megathrust earthquakes is important for both tsunami excitation and subsequent tsunami earthquake potential, but it is unclear whether frictional properties and/or fault structure determine the up-dip limit. A finite-fault slip model for the 2021 MW 8.2 Chignik, Alaska Peninsula earthquake obtained by joint inversion of seismic-geodetic data with model spatial extent constraints from the tsunami waves provides unusually good constraints on the up-dip edge of coseismic slip. Rupture initiated ∼35 km deep and propagated unilaterally northeastward with large-slip (up to 8.4 m) distributed over a depth range of 26 to 42 km beneath the continental shelf. Aftershocks concentrate up-dip of the coseismic slip around a strong megathrust reflector with high Coulomb stress change. The ∼25 km deep up-dip edge of slip strongly correlates with a change in plate interface reflectivity apparent in reflection profiles, indicating that a structural and frictional transition provided a barrier to shallower rupture. •Joint analysis of extensive observations to resolve the slip extent of the 2021 Chignik earthquake.•High-resolution slip is determined by iteration of the finite-fault inversion and tsunami predictions.•The 2021 earthquake ruptured a deeper portion of the Semidi segment with no shallow slip.•Complex physical state of the subduction zone controlled the up-dip limits of the rupture.•The 2021 rupture prompts us to reevaluate the rupture zone of the 1938 earthquake.