Abstract: A toilet over flow prevention system for installation into a toilet having a tank in fluid communication with a bowl and having a flapper style flush valve and a supplemental water supply. The over flow system includes a flapper solenoid mounted in the tank, for depressing the flapper valve into a closed position, a overflow sensor mounted in the bowl for sensing a over fill condition in the bowl, and a control box communicating with the over flow sensor and the flapper solenoid for operably signalling the flapper solenoid to depress the flapper valve into the closed position in the case of a over fill condition.