Supported noble metal nanoparticles (including nanoclusters) are widely used in many industrial catalytic processes. While the finely dispersed nanostructures are highly active, they are usually thermodynamically unstable and tend to aggregate or sinter at elevated temperatures. This scenario is particularly true for supported nanogold catalysts because the gold nanostructures are easily sintered at high temperatures, under reaction conditions, or even during storage at ambient temperature. Here, we demonstrate that isolated Au single atoms dispersed on iron oxide nanocrystallites (Au₁/FeO_x) are much more sinteringresistant than Au nanostructures, and exhibit extremely high reaction stability for CO oxidation in a wide temperature range. Theoretical studies revealed that the positively charged and surface-anchored Au1 atoms with high valent states formed significant covalent metal-support interactions (CMSIs), thus providing the ultra-stability and remarkable catalytic performance. This work may provide insights and a new avenue for fabricating supported Au catalysts with ultra-high stability.