In vibration fatigue, high-spatial-density experimental damage identification is hard to conduct. Fatigue damage is typically localized (in time and space) and loads can change direction with time. Thermoelasticity studies the interaction between temperature changes and elastic deformations in materials: minute changes in temperature can be related to the sum of the normal stresses, providing information about the multiaxial stress state. This research discusses the application of thermoelasticity in multiaxial criterion resulting in the equivalent uniaxial load. In this research, the thermoelasticity-based equivalent uniaxial load is related to the established theory on vibration-fatigue damage estimation in the spectral domain. The introduced thermoelasticity-based criterion is compared to the Equivalent von Mises stress criterion. Building on theoretical, numerical, and experimental research, this work examines the limitations of thermoelasticity-based criterion. Where the surface shear stresses are significantly smaller than the normal stresses, the numerical and experimental research shows promising results. Based on the introduced thermoelastic multiaxial criterion and with the recent progress in thermal imaging and signal processing, new possibilities for a close-to-real-time full-field fatigue-damage estimation open up.