The nonlinear viscoelastic–plastic responses of proton exchange membrane (PEM) under immersed condition at 30 and 70 °C are experimentally studied. For the description of the viscoelastic–plastic behavior of the material, Schapery's nonlinear viscoelastic model is used in combination with a nonlinear viscoplastic constitutive law. PEM's creep–relaxation behavior is predicted by the model with a constant duration at various stresses and a fixed stress level with different durations. The prediction results are found to agree quite well with experimental values at 30 °C. A step-stress input profile is conducted to validate the model and the result shows reasonable agreement with the experimental data indicating that the proposed model provides the capability to describe the characteristic of membranes in liquid water. However, this model cannot describe the creep behavior at 70 °C precisely mainly due to the distinct behavior of the membrane at the temperature. Thus, a modified model is developed and prediction values of the modified model at 30 and 70 °C are both found to be in good accordance with experimental results.