How to extend ultraviolet photocatalysts to the visible-light region is a key challenge for solar-driven photocatalysis. Herein, we show that ultraviolet ZnO photocatalysts can present high visible-light photocatalytic activity when combined with CuO quantum dots (QDs; <3 nm). Theoretical analysis demonstrates that the quantum size effect plays a key role in the photoactivity of the CuO/ZnO composite. For CuO QDs smaller than 3 nm, the separated charges could transfer from CuO QDs to the conduction bands of ZnO due to quantum splitting of the CuO energy level and phonon compensation for the difference in the conduction band minimum of CuO and ZnO; however, this process would not occur with the disappearance of the quantum size effect. Further structural analysis demonstrates that interfacial charge separation and transfer between ZnO and CuO dominate the photocatalytic processes instead of a single CuO or ZnO surface. Compared with ZnO-noble metal structures (e.g., ZnO-Ag or ZnO-Au), these ZnO-CuO QD composites present wider absorption bands, higher visible photocatalytic efficiencies, and lower costs.