Nitrogen fixation under ambient conditions remains a significant challenge. Here, we report nitrogen fixation by Ru single-atom electrocatalytic reduction at room temperature and pressure. In contrast to Ru nanoparticles, single Ru sites supported on N-doped porous carbon greatly promoted electroreduction of aqueous N-2 selectively to NH3, affording an NH3 formation rate of 3.665 mg(NH3) h(-1) mg(Ru)(-1) at -0.21 V versus the reversible hydrogen electrode. Importantly, the addition of ZrO2 was found to significantly suppress the competitive hydrogen evolution reaction. An NH3 faradic efficiency of about 21% was achieved at a low overpotential (0.17 V), surpassing many other reported catalysts. Experiments combined with density functional theory calculations showed that the Ru sites with oxygen vacancies were major active centers that permitted stabilization of *NNH, destabilization of *H, and enhanced N-2 adsorption. We envision that optimization of ZrO2 loading could further facilitate electroreduction of N-2 at both high NH3 synthesis rate and faradic efficiency.