A novel Cu2O/TiO2 composite photocatalyst structure of Cu2O nanospheres decorated with TiO2 nanoislands were synthesized by a facile hydrolyzation reaction followed by a solvent-thermal process. In this Cu2O/TiO2 composite photocatalyst, Cu2O served as the main visible light absorber, while TiO2 nanoislands formed heterojunctions of good contact with Cu2O, beneficial to the photoexcited electron transfer between them. Their band structure match and inner electrostatic field from the p-n heterojunction both favored the transfer of photoexcited electrons from Cu2O to TiO2, which effectively separated the electron-hole pairs. Photogenerated holes on Cu2O could react with water or organic pollutants/microorganisms in water to avoid accumulation on Cu2O because of the partial TiO2 nanoislands coverage, which enhanced their stability during the photocatalysis process. Their superior photocatalytic performance under visible light illumination was demonstrated in both the degradation of methyl orange and the disinfection of Escherichia coli bacteria. An interesting post-illumination catalytic memory was also observed for this composite photocatalyst as demonstrated in the disinfection of Escherichia coli bacteria in the dark after the visible light was shut off, which could be attributed to the transfer of photoexcited electrons from Cu2O to TiO2 and their trapping on TiO2 under visible light illumination, and their release in the dark after the visible light was shut off.