The advance of the synthesis of ammonia through nitrogen electroreduction under ambient conditions is seriously impeded by the lack of an efficient electrocatalyst that can facilitate multiple proton-coupled electron-transfer processes and suppress the competitive hydrogen evolution reaction required for a high nitrogen reduction reaction (NRR) selectivity. Herein, we demonstrate for the first time that boron-doped and nanostructured diamond can be used as an electrocatalyst for NRR, which affords a high NH3 yield rate of 19.1 μg h–1 cm–2, a Faradaic efficiency as high as 21.2%, and a stable operation of at least 8 days under ambient conditions. Substitutional boron atoms are illustrated to be capable of initiating the NRR active centers, and charge accumulation on the nanostructured diamond surface enables further enhancement of catalytic activity through reducing the reaction free energy for the rate-determining step of the NRR on certain surfaces.