To reduce its band gap of ZnO, a simple synthesis of ZnO–carbon composite spheres was developed using one-step solvothermal treatment of ethanol, Zn(Ac)2·2H2O, and iodine. During the process, alcoholysis of zinc ions and carbonization of ethanol occurred simultaneously with catalysis by iodine. Ethanol acts as both solvent and carbon precursor. The mass of iodine used in the reaction affected the shape and carbon amount-of-substance fraction of the composite. The photocatalytic activities of composites with different carbon amount-of-substance fractions were evaluated by the degree of degradation of aqueous solutions of rhodamine B at room temperature with visible light irradiation. The best photocatalytic activity was observed for a sample with a carbon amount-of-substance fraction of 30%. The formation mechanism involves iodine promotion of the oxidation of ethanol, which results in formation of carbon that self-assembles with the ZnO into the ZnO–C composite spheres.