An overstress viscoplasticity model is proposed to describe the rate-dependent behavior of a polymeric composite during loading and unloading. In the model, a three-parameter function is used to describe viscoplastic strain rate. In the loading stage, the equilibrium stress is determined using a multi-step relaxation test performed during loading. During the initial unloading stage, owing to the fact that the viscoplastic strain rate is still positive, the material still experiences “loading,” and the corresponding equilibrium stress is the equilibrium stress-strain curve for loading. In the second unloading stage, the viscoplastic strain rate becomes negative, and the material is in a true unloading mode for which the equilibrium stress is determined again using the multi-step “relaxation” test. The viscoplasticity model is found to be capable of capturing the characteristics of the rate-dependent loading and unloading behavior.