The presence of methane hydrate in soil pores alters the stress–strain and volumetric behaviour of soil. The extent of this alteration is affected by initial temperature and pore pressure when hydrate is formed inside the thermodynamically stable region. In this paper, a state-dependent critical state model is developed for methane hydrate-bearing sediments (MHBS) within the theoretical framework of bounding surface plasticity. A phase parameter is newly introduced into the constitutive model to account for the coupled effects of temperature and pore pressure on the mechanical behaviour of MHBS. This unique feature of the proposed model enables it to capture the behaviour of MHBS inside the methane hydrate stability region. A non-associated flow rule is adopted and a modified dilatancy expression is proposed considering the degree of hydrate saturation, phase parameter and stress state of MHBS. To verify the new model, computed results are compared to measured results of drained triaxial tests on MHBS with different morphologies and at different effective confining stresses, degrees of hydrate saturation and phase states inside the stability region. The comparison reveals that the model is capable of capturing the key features such as the evident strain-softening behaviour due to hydrate degradation and the change in stress–strain and volumetric behaviour of MHBS at different initial conditions inside the stability region.