A water balloon launching cannon is disclosed. The water balloon launching cannon includes a slotted barrel for rapid and easy loading and uses compressed air as the propelling force for firing the water balloon. Namely, the water balloon launching cannon includes an I-beam support upon which is mounted an air delivery system and the slotted barrel. For ease of disassembly, (1) the air delivery system includes quick-release mechanisms and (2) a plunger assembly inside the barrel is held by an easily removable pin. Further, a method of operating and a method using the water balloon launching cannon are disclosed.