In this study, we report sulfur-doped three-dimensional graphene (S-3DG) as a metal-free electrocatalyst for N₂ reduction reaction (NRR) under ambient conditions. Due to the high electron transport capacity and stable physicochemical properties of 3DG, it was utilized to improve the NRR catalytic performance dramatically. Hence, in 0.05 M H₂SO₄ the S-3DG achieved a remarkably large NH₃ yield of 38.81 μg_NH3 mg_cat⁻¹ h⁻¹ and a high faradaic efficiency of 7.72% at −0.6 V versus a reversible hydrogen electrode (RHE), which is superior to most non-metal catalysts. Notably, it shows such outstanding selectivity that no hydrazine by-products were detected and the electrochemical stability passed a long-term durability test.