A thermodynamically consistent model is formulated to depict the polarization hysteresis and the rate-dependent behavior of relaxation in ferroelectric materials. On the relaxation condition, the polarization gradually changes with time while the applied electric field is kept constant. The present model introduces internal state variables to represent such irreversible dissipation processes. Based on the first and second laws of thermodynamics, the evolution laws of the internal state variables consisting of constitutive equations are then derived through the definitions of the Helmholtz free energy and a dissipation potential. To verify the applicability of the constitutive model, numerical simulations are compared with experiment in the literature.