Based on the concept of generalized plasticity, this study proposes a constitutive model for describing the time-dependent behavior and wetting deterioration of sandstone. The proposed model (1) exhibits nonlinear elasticity under hydrostatic and shear loading; (2) follows the associated flow rule for viscoplastic deformation; (3) adopts acreep modulus that varies with stress ratios; and (4) considers the primary and secondary creep behaviors of rock. This model requires 13 material parameters, comprising 3 for elasticity, 7 for plasticity, and 3 for creep. The proposed model is first validated by comparing the triaxial tests of sandstone under different hydrostatic stress and cyclic loading conditions. Then, it is versatile in simulating the time-dependent behavior through a series of multi-staged creep tests.