In this paper, a thermoviscoplastic constitutive model for geomaterials is presented. The model is formulated based on two aspects: (i) the nonstationary flow surface theory, which is used to describe the viscous behaviour of geomaterials, and (ii) the thermoplasticity concept, which is adopted to introduce the effect of temperature on geomaterials. In the model, the yield surface evolves with the viscoplastic strain, viscoplastic strain rate and temperature. Thus, it allows the description of combined effects of strain rate and temperature on the mechanical behaviour of geomaterials. The model has been used to simulate various tests involving different thermomechanical loading paths, including non-isothermal constant rate of strain tests, cyclic heating and cooling tests, creep tests with temperature changes and stress relaxation tests. The simulation results agree well with measurements, demonstrating the capacity of the proposed model to reproduce the various thermoviscoplastic behaviour of geomaterials.