A path domain independent integral, S, that equals the energy release rate for an extending crack in a thermoinelastic field is presented. This paper develops the new parameter from its theoretical foundations in continuum mechanics, demonstrates that S can be calculated from finite element results and describes how S can be obtained from experiments. The S-integral is developed for simple uncoupled and linearized fully coupled quasi static thermoinelastic cases. Invoking thermoinelastic continuum mechanics linearized for small strain and small temperature changes, S emanates from a discrete Lagrangian describing the thermoinelastic system and Noether's theorem from classical field theory. S defines the force acting on an extending crack and represents a conservation law for a crack free body analogous to the Budiansky and Rice interpretation of the J-integral. The conservation law nature of S for a singularity free region is demonstrated by both computational and physical experiments. S can be calculated from finite element results via a two step postprocessing algorithm. Furthermore, S can be obtained from physical experiments. The S-integral offers a parameter to improve the understanding of the strength and reliability of materials subjected to thermomechanical loadings.