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Electrochemical conversion of N2–NH3 provides a green and sustainable alternative for artificial NH3 synthesis but requires effective electrocatalysts to promote N2 reduction reaction (NRR). This work reported the first application of N, S co-doped graphene (NSG) as an efficient metal-free NRR catalyst. It was found that the NSG exhibited a high NH3 yield of 7.7 μg h−1 mg−1 and Faradaic efficiency of 5.8% at − 0.6 V versus reversible hydrogen electrode under ambient conditions, which largely outperformed undoped and single-doped counterparts, and compared favorably to most reported metal-based NRR catalysts. The combination of density functional theory calculations and experiments revealed that the N, S co-doping could effectively improve the NRR of graphene by promoting the N2 adsorption and N≡N bond elongation, boosting the electron transfer and creating more defective/active sites. Furthermore, the NSG showed an excellent selectivity and stability, suggesting its great potential for efficient electrochemical synthesis of NH3. [ABSTRACT FROM AUTHOR]