This paper addresses a new, unified thermo-mechanical constitutive model for unsaturated soils through a coupled study. In the context of elasto-plasticity and the critical state theory, the model uses the concepts of multi-mechanism and bounding surface theory. This advanced constitutive approach involves thermo-plasticity of saturated and unsaturated soils. Bishop’s effective stress framework is adopted to represent the stress state in the soil. This stress is linked to the water retention curve, which is represented by an elasto-plastic model. Attention is focused particularly on the coupling relations inferred from this unified thermo-hydro-mechanical (THM) study. Finally, the theoretical aspects of the paper are supported by comparisons between numerical simulations and experimental results.