Industrial NH3 synthesis mainly relies on the carbon-emitting Haber-Bosch process operating under severe conditions. Electrocatalytic N2-to-NH3 fixation under ambient conditions is an attractive approach to reduce energy consumption and avoid direct carbon emission. In this communication, sulfur-doped graphene (S-G) is proposed as an efficient and stable electrocatalyst to drive the nitrogen reduction reaction (NRR) under ambient conditions. In 0.1 M HCl, this S-G attains a remarkably large NH3 yield of 27.3 μg h-1 mgcat.-1 and a high Faradaic efficiency of 11.5% at -0.6 and -0.5 V vs. a reversible hydrogen electrode, respectively, much higher than those of undoped G (6.25 μg h-1 mgcat.-1; 0.52%). Density functional theory calculations reveal that carbon atoms close to substituted sulfur atoms are the underlying catalytic active sites for the NRR on S-G, and the related NRR mechanism is also explored.