Cor triatriatum is a rare congenital heart defect that is associated with an increased risk for developing atrial fibrillation. We report a case of a healthy 38-year-old man who presented in decompensated heart failure and atrial fibrillation with a rapid ventricular response. A transthoracic echocardiogram (TTE) demonstrated severe biventricular dysfunction and dilatation in addition to cor triatriatum sinister. He was diuresed with resolution of his symptoms and spontaneously converted back to sinus rhythm. There is limited evidence in the literature surrounding anticoagulation and associated left ventricular dysfunction in the setting of cor triatriatum which posed difficult therapeutic decisions.