Noble-metal Au nanoparticles deposited on graphitic carbon nitride polymer (g-C3N4) photocatalyst by a facile deposition–precipitation method exhibited high photocatalytic activity for hydrogen gas production under visible-light irradiation. The Au/g-C3N4 nanocomposite plasmonic photocatalysts were characterized by X-ray diffraction spectroscopy, diffuse reflectance UV/Vis spectroscopy, FTIR spectroscopy, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, selected-area electron diffraction, X-ray photoelectron spectroscopy, photoluminescence spectroscopy, and photoelectrochemical measurements. We studied the effect of Au deposition on the photocatalytic activity of g-C3N4 by investigation of optical, electronic, and electrical properties. Enhanced photocatalytic activity of Au/g-C3N4 naocomposite for hydrogen production was attributed to the synergic mechanism operating between the conduction band minimum of g-C3N4 and the plasmonic band of Au nanoparticles including high optical absorption, uniform distribution, and nanoscale particle size of gold. The mechanism of te photocatalytic activity of the nanocomposite photocatalyst is discussed in detail. Deposition of Au nanoparticles on g-C3N4 was optimized and it was found that 1 wt % Au-loaded g-C3N4 composite plasmonic photocatalyst generated a photocurrent density of 49 mA cm−2 and produced a hydrogen gas amount of 532 μmol under visible light, which were more than 3000 times higher and 23 times higher, respectively, than the values of neat g-C3N4.