Legionella is an opportunistic premise plumbing pathogen that can be present in municipal and other water supplies. Building water systems may provide conditions (such as low flow, water hardness, low disinfectant residual levels and optimal temperature) that accelerate Legionella growth to levels that may result in an increased risk to public health. The standard disinfection of water systems (periodic overheating of water and chlorination) in the interest of prevention of Legionnaires' disease have often proved to be inefficient. It is therefore necessary to develop new approaches for removing Legionella from water systems. One of the new methods is antimicrobial photodynamic therapy (aPDT), which includes the combined activity of a photosensitizer (PS), molecular oxygen and visible light of appropriate wavelength to create singlet oxygen (1O2) and other oxygen reactive species (ROS) leading to the oxidation of numerous cellular components and cell death. In this study, a newly synthesized cationic, amphiphilic porphyrin TMPyP3-C17H35, was tested against Legionella in tap water. The minimal effective concentration (MEC) of PS photoinactivation test and PS uptake assay in sterile tap water were explored to determine the anti-Legionella activity. The complete inactivation of Legionella in sterile tap water was achieved with 0.024 μM of the PS. Also, the tested PS was found to be very effective in reducing Legionella growth in the sterile tap water and photoinactivation was dose-dependent. The tested PS binds well to the bacterial cell, after only 10 minutes of incubation in the dark. In conclusion, these studies indicate that TMPyP3-C17H35 is highly efficient in aPDT which leads to reducing Legionella growth in sterile tap water, and these results suggest that cationic amphiphilic photosensitizers may have a broader application in the photoinactivation of bacterial cells implicated in water disinfection.