AbstractElectroreduction of nitrite to ammonia (NO2RR) holds great promise for concurrently achieving efficient NH3 electrosynthesis and wastewater purification. This study first develops Ni1Ru single‐atom alloys as an efficient and robust NO2RR catalyst. Extensive experiments and theoretical computations reveal that isolated Ni atoms electronically couple their adjacent Ru atoms to render Ni1Ru with high thermodynamic stability, boosted NO2−‐to‐NH3 hydrogenation energetics and suppressed hydrogen evolution. As a result, Ni1Ru assembled in a flow cell shows an exceptional NH3 yield rate of 37.5 mg h−1 cm−2 with an NH3‐Faradaic efficiency of 93.9% at a high current density of 312.3 mA cm−2, representing one of the highest NO2RR performances ever reported.