By utilizing stable thiol self-assembled monolayers (SAMs) on gold (Au) nanosheets, we investigated the temperature dependence of the amount of resistivity increases Δρad by SAM modification. Δρad increased with increasing temperature from 5 to 375 K in all the measured nanosheets for various thicknesses (9–20 nm). We consider from the DFT calculation that the temperature dependence originates from unoccupied states of sulfur atoms above the Fermi level of Au. The available unoccupied states increase at higher temperatures, which enhances scattering rates and thus increases the resistivity. Furthermore, we calculated the temperature dependence of Δρad using the Bloch–Grüneisen theory and Fuchs–Sondheimer (FS) model. Based on the results, we conclude that for ultra-thin metal films, the temperature-dependent specularity parameter PT which represents the fraction of electrons scattered specularly should be introduced into the conventional FS model to predict Δρad over a wide temperature range.