Gallium zinc oxynitride (GaN:ZnO) solid solution is one of the most promising visible-light activated photocatalysts due to high stability in water splitting reaction conditions and controllable band edge potentials. To further enhance the activity of this photocatalyst by facilitating the photo-induced charges separation, a nanocomposite of GaN:ZnO solid solution photocatalyst and graphene oxide (GO) nanosheets was fabricated through facile ultrasonication. Structural, optical, and electrochemical characterization of the prepared composite revealed the effective interaction between composite components and significant improvement in charge separation. The synthesized composite with a defined amount of GO demonstrated superior activity for overall water splitting under visible light irradiation with significantly higher apparent quantum efficiency.