Removing excess nitrate (NO3–) from waste streams has become a significant environmental and health topic. However, realizing highly selective NO3– conversion toward N2, primarily via electrocatalytic conversions, has proven challenging, largely because of the kinetically uncontrollable NO3–-to-NO2– pathway and unfavorable N–N coupling. Herein, we discovered unique and ultra-high electrocatalytic NO3–-to-NO2–activity on oxide-derived silver (OD-Ag). Up to 98% selectivity and 95% Faradaic efficiency (FE) of NO2– were observed and maintained under a wide potential window. Benefiting from the superior NO3–-to-NO2–activity, further reduction of accumulated NO2– to NH4+ was well regulated by the cathodic potential and achieved an NH4+ FE of 89%, indicating a tunable selectivity to the key nitrate reduction products (NO2– or NH4+) on OD-Ag. Density functional theory computations provided insights into the unique NO2– selectivity on Ag electrodes compared with Cu, showing the critical role of a proton-assisted mechanism. Based on the ultra-high NO3–-to-NO2– activity on OD-Ag, we designed a novel electrocatalytic–catalytic combined process for denitrifying real-world NO3–-containing agricultural wastewater, leading to 95+% of NO3– conversion to N2 with minimal NOX gases. In addition to the wastewater treatment process to N2 and the electrochemical synthesis of NH3, NO2– derived from electrocatalytic NO3– conversion can serve as a reactive platform for the distributed production of various nitrogen products.