Electrochemical synthesis of NH3 is a carbon-free alternative to the traditional Haber-Bosch process. The challenge with nitrogen reduction reaction (NRR) to NH3 is cleavage of the inert N≡N triple bond of nitrogen gas. Obtaining NH3 from environmental pollutants, such as nitrates or nitrites, is a more practical route than NRR. However, reduction of nitrates or nitrites to ammonia is currently hampered by modest Faradaic efficiencies, typically below 10 %. Here, we report a novel heterogeneous catalyst based on iron (Fe) single-atoms supported on two-dimensional MoS2 (Fe-MoS2) for the nitrate reduction reaction (NO3RR). We have found that Fe-MoS2 exhibits remarkable performance with a maximum Faradaic efficiency of 98 % for NO3RR to NH3 at an overpotential of -0.48 V vs. the reversible hydrogen electrode (RHE) as confirmed by our isotopic nuclear magnetic resonance (NMR) analyses. Density function theory (DFT) calculations reveal that the enhanced selectivity for the production of NH3 from single Fe atoms supported on MoS2 is attributed to a reduced energy barrier of 0.38 eV associated with de-oxidation of *NO to *N – the usual potential limiting step in NO3RR. We assembled our catalyst in a two-electrode electrolyzer coupled to an InGaP/GaAs/Ge triple-junction solar cell to demonstrate a solar-to-ammonia (STA) conversion efficiency of 3.4 % and a yield rate of 0.03 mmol h-1 cm-2 equivalent to 510 µg h-1 cm-2. Our results open new avenues for design of single-atom catalysts (SAC) for the realization of solar-driven ammonia production.