Developing cost-effective, high-performance nitrogen reduction reaction (NRR) electrocatalysts is required for the production of green and low-cost ammonia under ambient conditions. Here, a strategy is proposed to adjust the reaction preference of noble metals by tuning the size and local chemical environment of the active sites. This proof-of-concept model is realized by single ruthenium atoms distributed in a matrix of graphitic carbon nitride (Ru SAs/g-C 3 N 4 ). This model is compared, in terms of the NRR activity, to bulk Ru. The as-synthesized Ru SAs/g-C 3 N 4 exhibits excellent catalytic activity and selectivity with an NH 3 yield rate of 23.0 µg mg cat −1 h −1 and a Faradaic efficiency as high as 8.3% at a low overpotential (0.05 V vs the reversible hydrogen electrode), which is far better than that of the bulk Ru counterpart. Moreover, the Ru SAs/g-C 3 N 4 displays a high stability during five recycling tests and a 12 h potentiostatic test. Density functional theory calculations reveal that compared to bulk Ru surfaces, Ru SAs/g-C 3 N 4 has more facile reaction thermodynamics, and the enhanced NRR performance of Ru SAs/g-C 3 N 4 originates from a tuning of the d-electron energies from that of the bulk to a single-atom, causing an up-shift of the d-band center toward the Fermi level.can maximize metal utilization. Since SACs have unique catalytic sites, they usually exhibit a distinct catalytic selectivity as compared to their nanoclusters or nanoparticle counterparts. [2] For example, single atomic Pt immobilized in the surface of Ni nanocrystals shows a higher activity and chemoselectivity toward the hydrogenation of 3-nitrostyrene. [3] Isolated Co single-site catalysts anchored on a N-doped porous carbon nanobelt exhibits an excellent catalytic performance for oxidation of ethylbenzene with 98% conversion and 99% selectivity, whereas the Co nanoparticles are essentially inert. [4] Moreover, atomic Ni-anchored covalent triazine framework has a remarkable selectivity for the conversion of CO 2 to CO, with a Faradaic efficiency (FE) of > 90% over the range of −0.6 to −0.9 V versus the reversible hydrogen electrode (RHE). [5] In view of these reported works, it is evident that the size of metal particles is a key factor in determining their catalytic performance, and decreasing the size offers an intriguing opportunity to alter the activity and selectivity of these metal catalysts. SACs, as the limit of size reduction, hold great potential to achieve high activity and selectivity in catalytic reactions.Recently, the electrocatalytic N 2 reduction reaction (NRR) in aqueous electrolytes for synthesizing ammonia at ambient