Abstract: A fuel overflow prevention system for substantially eliminating fuel spillage both due to over-filling during fueling and due to fuel expansion in fuel tanks. The device includes a fuel discharge preventor and a nozzle adapter that work together during re-fueling to allow displacement by fuel of gases within the fuel tank until such time that the fuel reaches a predetermined level in the tank. The predetermined level is chosen through the geometry of the fuel discharge preventor so as to provide sufficient space for thermal expansion of the fuel without subsequent fuel spillage. The fuel discharge preventor also contains a valve that closes upon attainment of the predetermined fuel level and, together with the nozzle adapter, causes a pressure build-up within the filling fuel tank so as to positively actuate the automatic turn-off features of the standard fuel pump nozzle utilized.