A composite photocatalyst composed of carbonaceous material and g-C3N4 is fabricated by a simple thermal treatment of melamine (hydrothermally treated melamine) with cellulose acetate. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and elemental analysis data reveal the formation of carbonaceous material in the composites. The photocatalytic activity was evaluated by the H-2 liberation rate from water splitting under visible-light irradiation. The optimized composite, which is prepared from the precursor of hydrothermally treated melamine with cellulose acetate, shows 35 times higher photocatalytic activity than bulk g-C3N4 (melamine as the precursor) and a quantum efficiency of 1.05% (0.01 g of catalyst) under irradiation at 420 nm. The enhancement of the photocatalytic activity is ascribed to the unique electronic properties of the carbonaceous material, the enlarged specific surface area and the improved light absorption.