Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new dataset of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River-parallel near the surface to river-perpendicular in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ~13 and ~26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust of an estimated apparent friction coefficient of ~0.5. Finally, depth-dependent b-values confirm the rheological difference between upper and middle crust in the CSZ.