This work focuses on the synthesis of composite materials based on reduced graphene oxide reinforced with zinc oxide. The zinc precursor concentration was varied, and the effect on the structural, morphological, chemical and photocatalytic properties of the resultant composites was studied. The results show that possible variations in the kinetics of the crystal growth due to the Zn precursor concentration can change the final sizes and morphologies of the ZnO structures anchored in the composite. Two different morphologies, ZnO flowers (rGO/ZnO-flowers) and ZnO rods (rGO/ZnO-rods), were obtained and analyzed. Both composites were synthesized using a microwave-assisted hydrothermal route under the same experimental conditions. They were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and UV-VIS DRS. Both composites presented high adsorption (60–70%) and photocatalytic degradation (ca. 100%) of aqueous methylene blue, which was explained in terms of the excellent coupling between the rGO sheets and the different ZnO morphologies.