A system for feeding and sanitizing the water circuits of dental units comprises the following, combined with each other: at least one second branch (7) connected to and leading into a first branch (3) for feeding a second fluid, as an alternative to a first fluid from a first feed line (1); the second branch (7) and the first feed line (1) being equipped with corresponding second cutoff means (8) and third cutoff means (9) designed to switch between feeding the first branch (3) with the first fluid amd feeding it with the second fluid when required; a first container (10) for housing handpieces (5) during sterilization, disinfection, flushing or cleaning treatments on the water circuit, and switching means (11), connected to control means (12), which can be activated by an external command or automatically upon the occurrence of certain events on the dental unit, depending on the treatment to be carried out, and which are designed to perform combinations and/or manual or automatic sequences, even repeated two or more times in succession, of opening and closing operations on first cutoff means (6), second cutoff means (8) and third cutoff means (9) in such a way as to enable the first branch (3) to perform treatment cycles with the first fluid (2) and/or alternately with the second fluid conveyed by the second branch (7).