In this work, graphitic carbon nitride (g-C3N4) supporting finely dispersed Au nanoparticles is developed as a simple and efficient photocatalyst for H2O2 production under visible light irradiation. Au nanoparticle cocatalyst remarkably enhances photocatalytic activity of C3N4 for H2O2 production. Due to the inert nature to catalyze the decomposition of H2O2, Au/C3N4 also exhibits stable H2O2 evolution rate. It is of great interest that the maximal H2O2 production activity is reached at the loading amount of Au as low as 0.01%, revealing the great catalytic efficacy of highly dispersed Au cocatalyst during the photocatalytic H2O2 synthesis and the possibility to produce concentrated H2O2 using C3N4 with extremely low Au loading amount. The in situ electron spin resonance studies reveal that the H2O2 is produced through direct 2e− oxygen reduction over Au/C3N4 photocatalyst.