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Decorating photocatalysts with noble metal nanoparticles (e.g., Pt) often increases the catalysts’ photocatalytic activity and biomedical properties. Here, a simple and inexpensive method has been developed to prepare a Pt-Ag₃PO₄/CdS/chitosan composite, which was characterized and used for the visible light-induced photocatalytic and antibacterial studies. This synthesized composite showed superior photocatalytic activity for methylene blue degradation as a hazardous pollutant (the maximum dye degradation was observed in 90 min of treatment) and killing of Gram positive bacterial (<i>Staphylococcus aureus</i> and <i>Bacillus cereus</i>) as well as Gram negative bacteria (<i>Klebsiella pneumoniae</i>, <i>Salmonella typhimurium</i>, <i>Escherichia coli</i>, and <i>Pseudomonas aeruginosa</i>) under visible light irradiation. The antibacterial activity of CdS, CdS/Ag₃PO₄, and Pt-Ag₃PO₄/CdS/chitosan against <i>E. coli</i>, <i>Pseudomonas aeruginosa</i>, <i>Salmonella typhimurium</i>, <i>Klebsiella pneumoniae</i>, <i>Staphylococcus aureus</i>, and <i>Bacillus cereus</i> showed the zone of inhibition (mm) under visible light and under dark conditions at a concentration of 20 µg mL⁻¹. Furthermore, the cell viability of the CdS/chitosan, Ag₃PO₄, Ag₃PO₄/CdS/chitosan, and Pt-Ag₃PO₄/CdS/chitosan were investigated on the human embryonic kidney 293 cells (HEK-293), Henrietta Lacks (HeLa), human liver cancer cell line (HepG2), and pheochromocytoma (PC12) cell lines. In addition, the results indicated that the photodegradation rate for Pt-Ag₃PO₄/CdS/chitosan is 3.53 times higher than that of CdS and 1.73 times higher than that of the CdS/Ag₃PO₄ composite. Moreover, Pt-Ag₃PO₄/CdS/chitosan with an optimal amount of CdS killed large areas of different bacteria and different cells separately in a shorter time period under visible-light irradiation, which shows significantly higher efficiency than pure CdS and other CdS/Ag₃PO₄ composites. The superb performances of this composite are attributed to its privileged properties, such as retarded recombination of photoinduced electron/hole pairs and a large specific surface area, making Pt-Ag₃PO₄/CdS/chitosan a valuable composite that can be deployed for a range of important applications, such as visible light-induced photocatalysis and antibacterial activity.