Abstract: A system for use with a normally open manually actuated water valve, or any other suitable manually actuated device, for automatically closing the valve, or device, upon sensing water. The system includes at least one water sensor electrically connected to a motorized actuator mounted on the valve, or device, whereby a signal is transmitted either by electrical conductor, or in an alternate embodiment by RF transmission, to the motorized actuator upon sensing water, whereby said manual valve is automatically actuated to its closed position. The system may be installed on an otherwise conventional manual shut-off valve, without replacing the valve or otherwise interrupting flow through the valve.