<p>An isotropic damage model based on microcrack density evolution is proposed to describe the mechanical behavior of quasi-brittle geomaterials under compression stress. In this model, the damage variable is defined by the crack density, and frictional sliding between the surfaces of closed microcracks is considered the mechanism responsible for macroscopic plastic deformation. It is assumed that closed microcracks have no influence on the macroscopic elastic properties of the geomaterials, so the elastic response remains unaffected during damage evolution. To accurately capture the stress–strain relationship of geomaterials under compressive loading, the model adopts a generalized friction criterion as the yield function and introduces a strain energy release rate-based damage evolution law, while also accounting for the coupling effects between damage and plastic behavior. Moreover, to describe the dilatancy behavior observed in brittle geomaterials under compression, a non-associated plastic flow rule is adopted to improve the realism of the mechanical response. The proposed model’s performance is validated for two different quasi-brittle geomaterials, granite and concrete, under different confining pressures. The model accurately captures the evolution of strength and strains, including axial, lateral, and volumetric strains, for quasi-brittle geomaterials.</p>