The invention concerns a device for sealing a nitrogen oxide reducing agent tank of a motor vehicle, the device comprising a reducing agent tank spout (10) and a reducing agent tank cap (12), characterized in that it comprises an extension (16, 17, 18) extending from the cap (12), which extension (16, 17, 18) comprises fastening means intended to cooperate with a fuel tank cap (22) to retain the extension (16, 17, 18) against an opening of the reducing agent tank cap (12).
