AbstractWe monitored the seismohydromechanical rock mass response to high‐pressure fluid injection during a decameter‐scale hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, Switzerland. Time series recorded at two pressure monitoring locations show abrupt pressure increases that change in amplitude and appearance between subsequent stimulation cycles. Induced seismicity correlates with the propagation of one of these pressure fronts. Deformation data along the same shear zone shows permanent fracture dislocation preceded by strong transient fracture opening. We interpret these observations as nonlinear pressure diffusion along flow channels that reorganize in response to hydromechanical effects during stimulation. Combining these observations with the in situ stress field estimated before stimulation, we argue that the underlying hydromechanical processes involve mixed‐mode stimulation with both Mode I and II/III fracture dislocation.