A unique solution is proposed to the problem of how thermodynamic processes between thermodynamic systems at relative rest “appear” to a moving observer. Assuming only transformations for entropy, pressure, and volume and the invariance of the “fundamental thermodynamic equation,” one can derive transformations for (thermodynamic) energy and temperature. The invariance of the first and second laws entails transformations for work and heat. All thermodynamic relations become Lorentz-invariant. The transformations thus derived are in principle equivalent to those of Einstein and Planck, except that our expressions for energy and work do not include the mass motion energy. This results in a simpler formulation and endows our transformations (especially that of temperature) with a straightforward physical interpretation.