Abstract We present a theoretical framework to quantify the equivalent Biot coefficient tensor of fractured rock masses based on the properties of both the intact porous matrix and the fracture network. Our results show that the presence of dominant fracture sets produces an anisotropic equivalent Biot tensor, with larger values in the direction perpendicular to the fractures than parallel to them. This anisotropy has a marked impact on the subsurface hydro‐mechanical (HM) response to fluid injection and pressurization: it induces deviatoric stress variations which amplifies failure potential and rotates the effective stress tensor. Because such effects may promote earlier fault reactivation and seismicity—often at lower pressure buildup and on fault orientations that would not otherwise be considered critical—accurate quantification of the equivalent Biot tensor is essential for reliable assessment of failure potential in subsurface fluid‐injection operations.