Cardiac hydatid cyst is known to be a rarely occurring disease. The appearance of large cysts in the interventricular septum in childhood is even more rare. Although such cysts are usually asymptomatic, they can behave like valvular disorders, depending on their location. In addition, cardiac hydatid cysts originating from the interventricular septum carry the risk of rupturing into both ventricular cavities, which may lead to fatal complications. Thus, early surgical treatment is of extreme importance. We describe the case of a 7-year-old girl with a cardiac hydatid cyst that originated in the interventricular septum.