The features of deuterium adsorption on the surface of gold nanoparticles deposited on highly oriented pyrolytic graphite (HOPG) were determined. The results showed that deuterium adsorption on gold nanoparticles takes place at room temperature. The results also showed that the filling of the nanoparticles’ surfaces with the adsorbate occurs from the graphite–gold interface until the entire surface is covered by deuterium. The results of quantum chemical simulations are used to explain the experimental data. A simple model of the observed effects is proposed.