A flood detection and valve shutoff device which comprises a water sensitive sensor. A mechanism is for retaining the water sensitive sensor on a floor near a water heater. Another mechanism is for biasing a valve in a water inlet pipe of the water heater to a closed position. A lanyard in a taut condition extends between the water sensitive sensor and the biasing mechanism, so as to keep the valve in an open position. When the water sensitive sensor detects water on the floor the lanyard will be released from the water sensitive sensor, causing the biasing means to move the valve from the open position to the closed position.