The Ag/ZnO/g-C3N4 composite photocatalyst, which was synthesized by thermal polymerization of a melamine precursor combined with solvothermal reaction, was used as a visible-light-driven biocide toward Escherichia coli (E. coli). The Ag/ZnO/g-C3N4 composite displayed considerably increased visible-light-driven bactericidal activity compared to g-C3N4, Ag/g-C3N4, and ZnO/g-C3N4 powders. The photocatalytic destruction of E. coli cells was verified by fluorescence-based cell live/dead analysis and SEM. The mechanism of increased bactericidal effect was studied by UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, photocurrent response, and electrochemical impedance spectroscopy (EIS). The enhanced photocatalytic bactericidal effect was attributed to the matching conduction band levels of g-C3N4 and ZnO and the surface plasmon resonance (SPR) effect of Ag nanoparticles, which resulted in enhanced adsorption of visible light, reduced recombination of free charges, and rapid separation and transportation of photogenerated electrons-holes. The disinfection mechanism was investigated using different chemical scavengers and electron spin-resonance spectroscopy (ESR), and indicating the important role of h(+) and H2O2. Furthermore, taking into account the bulk availability and better bactericidal effect of Ag/ZnO/g-C3N4, this work provides a new pathway to develop advanced materials for water disinfection.