An overfill prevention valve that is testable to ensure proper functionality of the valve. More particularly, in one embodiment the invention is an overfill prevention valve system including an overfill prevention valve having a blocking component. The blocking component is movable between a closed position, wherein the blocking component generally blocks fluid from flowing through the valve, and an open position, wherein the blocking component generally allows fluid to flow through the valve. The valve further including a float operatively coupled to the blocking component. The system additionally has an actuator that is operable to directly or indirectly move the blocking component to at least one of its open or closed positions to thereby enable testing of the overfill prevention valve.