A toilet overflow prevention device including an overflow valve assembly, a control valve and a water level sensor. The overflow valve assembly is configured to selectively permit a flow of water through the valve assembly from the tank to the bowl of an associated toilet. The water level sensor is configured to provide a control signal in response to detecting an above-normal water level within the bowl of the toilet. The control valve is configured to actuate the overflow valve assembly in response to a control signal from the water level sensor. Preferably, the overflow valve assembly is separate from the primary flush valve of the toilet and is located between the primary flush valve and the bowl of the toilet.