The electrochemical nitrogen reduction reaction (e-NRR) is envisaged as alternative technique to the Haber–Bosch process for NH₃ synthesis. However, how to develop highly active e-NRR catalysts faces daunting challenges. Herein, a viable strategy to manipulate local spin state of isolated iron sites through S-coordinated doping (Fe_SA-NSC) is reported. Incorporation of S in the coordination of Fe_SA-NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t_2g6 e_g1), which is beneficial for facilitating e_g electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10 mA cm⁻² can be achieved, together with a high NH₃ selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the Fe_SA-NSC after the S doping accelerates the N₂ activation and protonation in the rate-determining step of *N₂ to *NNH.