This paper introduces a novel smoothing gradient-enhanced thermo-mechanical damage model to capture complex thermal crack patterns. To describe the adiabatic property of the damaged region, the scalar variable indicating the deterioration level of material, the damage variable, is employed for the heat conduction balance equation. In essence, the local heat flux and heat capacity are modified to present a decrease in heat flow. Formulations for steady state and transient heat transfer are given. The accuracy and efficiency of the coupled smoothing gradient damage model are illustrated through the numerical experiments for steady state and transient crack growth (e.g., quenching test), where the predicted crack patterns induced by thermo-mechanical loading are in good agreement with experimental data and other reference numerical solutions.