We have discovered that NO2 is reduced to NO at 77 K by hydrogen precovered gold in vacuum. Here, we investigate the partial reduction of NO2 to NO on an atomic-hydrogen populated model gold catalyst for a more fundamental understanding of the surface chemistry of hydrogenation. Gold-based catalysts have been found to be active for many hydrogenation reactions, but few related fundamental studies have been conducted. Our experimental results reveal a high catalytic activity for gold: indeed, NO2 is reduced to NO with 100% conversion and 100% selectivity at temperatures lower than 120 K. Density functional theory calculations and reflection-absorption infrared spectroscopy measurements indicate that HNO2 and N2O3 are intermediates which are highly dependent on surface hydrogen concentrations; subsequent hydrogenation of HNO2 and dissociation of N2O3 upon annealing induces the production of NO and H2O.