Constitutive models developed for understanding the hydromechanical behavior of unsaturated soils (UNSAT) that should be considered as three-phase media have been studied in the last thirty years. While some of these models capture the two-way coupled behavior in which mechanical and hydraulic components factor in together, others take into account the hydraulic hysteresis observed only in UNSAT water retention behavior in addition to the stress-strain relationship. In this study, a one-way coupled model for UNSAT is developed, where mechanical and hydraulic effects for the soil are considered simultaneously. One-way coupling is provided by the model’s ability of predicting the change in degree of saturation of pore water during total stress loading. This directly correlates with the estimation of amount of pore water leaving the soil for drained condition as well as the decrease in matric suction for undrained condition. The model uses independent stress variables following the formulation of the classical plasticity theory for the elasto-plastic stress-strain relationship of the solid skeleton. The changes in water content is calculated using the volumetric strain with a novel equation derived in this study. The hydraulic part of the model requires only three new model parameters, all of which can be obtained from a single isotropic compression test by measuring the water content. Model results are calibrated with UNSAT triaxial tests at both constant suction and at constant water content conditions. Initial predictions are encouraging for future use of the proposed constitutive model in boundary value problems.