We tune the valence state of single Au atoms anchored on CeO₂(100) by treating the catalyst in H₂ at different temperatures and obtain a series of Au₁/CeO₂(100). The transition from Au₁^+0.9 to Au₁^+0.3 leads to an enhancement of the CO oxidation activity of Au₁/CeO₂(100) by one order of magnitude. This work is of significance for an in-depth understanding of reaction mechanisms and rational design of high-performance catalysts.