The authors have shown that nanogranular indium tin oxide (ITO) films, deposited by spray pyrolysis on a silicon substrate, demonstrate ultralow thermal conductivity κ ∼ 0.84 ± 0.12 Wm⁻¹ K⁻¹ at room temperature. This value is approximately by one order of magnitude lower than that in bulk ITO. The strong drop of thermal conductivity is explained by the nanogranular structure and porosity of ITO films, resulting in enhanced phonon scattering on grain boundaries. The experimental results were interpreted theoretically, employing the Boltzmann transport equation approach for phonon transport and filtering model for electronic transport. The calculated values of thermal conductivity are in reasonable agreement with the experimental findings. The presented results show that ITO films with an optimal nanogranular structure may be prospective for thermoelectric applications.