A 2-yr-old male Siberian Tiger (Panthera tigris altaica) died during a fight with a conspecific. At necropsy, significant abnormalities included severe cardiomegaly with cardiac malformations consisting of a large atrial septal defect, a membranous ventricular septal defect, overriding aorta, and stenosis of the pulmonary valve with secondary concentric hypertrophy of the right ventricle. Endocardiosis of the mitral valve was also noted. To the authors' knowledge, this is the first report of a pentalogy of Fallot in a large felid that resulted in sudden death.