A constitutive relation for thermoviscoelastic-plastic material is derived in terms of internal variables. Three types of internal variables-plastic strain, the hardening parameter, and a variable exhibiting viscous effects-are introduced, and the thermoviscoelastic strain and rate-dependent plastic strain are given in the framework of the thermodynamics of solids. As an example, stress-strain relations are calculated for a material subjected to uniaxial tension, and the rate-dependent behavior is discussed.