The atrioventricular canal defect is a rare congenital cardiac anomaly that exists in a partial and complete form. The cardiac defects that may occur in the septum primum of the atrium, the ventricular septum, and the atrioventricular valves are secondary to abnormal endocardial cushion development in the heart. This case report describes a 6 week old female thoroughbred foal with a complete atrioventricular canal defect. Electrocardiography, echocardiography, and intracardiac pressure measurements were utilized to characterize the defect and its hemodynamic consequences. An atrial septal defect, ventricular septal defect, and a common atrioventricular valve were identified. The common AV valve consisted of five leaflets with two free-floating bridging leaflets in the center. The foal was euthanized and a necropsy was performed, which confirmed the anatomic anomalies. Histology of lung and liver samples illustrated the presence of congestive heart failure and pulmonary hypertension. This article will highlight the abnormal embryology, anatomy, and pathophysiology associated with the complete atrioventricular canal defect and its ensuing clinical signs.