Aims:  To investigate infra‐specific spatio‐temporal dynamics of a hospital water network Pseudomonas aeruginosa population. To infer the origin of water network isolates and assess their potential health hazard. Methods and Results:  168 P. aeruginosa strains were isolated from tap waters and swabs of tap nozzle aerators of a hospital unit, over 2 years, and from rectal swabs and nosocomial infections. Genetic diversity among this collection was assessed by pulsed field gel electrophoresis of SpeI restricted genomic DNA. Virulence gene sets, biofilm properties, and hypochlorite resistance were analysed. Exactly 68% of the water samples and 74% of the tap nozzle aerators harboured P. aeruginosa. The strains were divided into 22 clonal lineages, with one dominant clone shown to have been involved in a nosocomial infection. Conclusions:  An important turnover among the P. aeruginosa hospital population was observed. Some clonal lineages were found to persist, spread in the unit, and diversify into clonal complexes. Rectal carriage appeared an important source of contamination of the water network. Significance and Impact of the Study:  High P. aeruginosa infra‐specific population diversity suggested a broad ability in colonizing water networks but persistence analysis indicated a strong selection leading to the emergence of dominant clones.