Electrochemical nitrogen fixation under mild conditions is highly demanded yet remains a grand challenge. Herein we report metal-free electrocatalysis of aqueous N2 reduction to produce NH3 over reduced graphene oxide (DrGO) with tuned defects. The defect sites in DrGO, consisting of unsaturated carbon [single vacancy (SV), double vacancy (DV), and –COOH], were examined, and showed an improved NH3 selectivity due to the strong binding of N2 instead of H. In addition to improved selectivity, the calculated free energies for N2 reduction reaction at DrGO-COOH and DrGO-DV sites suggest that the thermodynamic overpotentials of these metal-free catalysts are comparable to the most efficient transition metal-based catalysts reported thus far. Our nonmetallic and dopant-free catalysts can convert N2 to NH3 at a faradaic efficiency of up to 22.0% at −0.116 V (versus the reversible hydrogen electrode vs. RHE) in 0.1 M HCl and 10.8% at −0.166 V (vs. RHE) in 0.1 M KOH, surpassing most earlier reported catalysts. An NH3 formation rate exceeding 7.3 μg h−1 mg−1 was achieved at low overpotentials in both acidic and alkaline environments, comparable to the values shown by metal electrocatalysts under similar conditions.