In this paper, a thermodynamic approach is presented to model coupled fluid transport, heat transfer, long-term deformation and damage in polymeric materials. The well-known Gibbs free energy is expressed as a functional of stress, temperature and fluid concentration with damage being introduced as an internal state variable. Constitutive equations for nonlinear viscoelastic materials in hygrothermal environments are derived in memory functional forms. The kinetics of damage evolution induced by stress, temperature and fluid is described by a damage function with thermodynamic driving force. Governing equations for mass and heat transfer are obtained from transport laws relating fluid and heat fluxes to gradients of chemical potential difference and temperature. A superposition principle of time, temperature, fluid concentration, stress, and aging is proposed so that long-term property functions may be derived from momentary master curves by horizontal and vertical shifting. The approach provides a theoretical framework for evaluating longterm behavior of polymeric materials in hygrothermal environments from short-term experiments.