In this work, the results of the application of organic expanded porphyrins in the disinfection of water by the photodynamic inactivation (PDI) technique are presented. The photoinactivation properties of a novel, expanded porphyrin, namely 20-(4-carboxyphenyl)-2,13-dimethyl-3,12-diethyl-(22π) pentaphyrin (PCCox), were tested in the disinfection of water using Staphylococcus aureus as a Gram-positive bacteria model. The data showed that PCCox was effective against S. aureus bacteria at nanomolar concentrations. The variation with irradiation time and concentration was studied using both a multi-LED monochromatic light (λ = 470 nm) and an incandescent light bulb with a wide emission spectrum. A PCCox dosage of 5 μM was sufficient to achieve a 99.997% abatement of S. aureus within 1 h of 40 W/m2 irradiation with monochromatic light (λ = 470 nm), whereas under the same conditions using irradiation with white light, the abatement was 99.9997%.