Congenital heart disease (CHD) occurs uncommonly in cats but has potentially life-altering ramifications. This paper describes a case of two different congenital heart conditions, atrial septal defect (ASD) and cor triatriatum sinister (CTS), concurrently present in a 9 year old Domestic Shorthair Cat. Differential diagnoses for a left parasternal systolic murmur, including both pathological and non-pathological causes, are discussed. The etiology and embryology for ASDs and CTS are reviewed, as well as the standard means of diagnosing and treating each condition. The pathophysiology of Eisenmenger Syndrome, a common sequelae to septal defects, is also discussed