Cor triatriatum sinister is a rare congenital heart disease and rarely found in adults. The authors describe an asymptomatic 20-year old man presenting with heart murmur by check up. Transthoracic and transesophageal echocardiogram demonstrate atrial septal defect (ASD) secundum 26 mm and cor triatriatum sinister with 20 mm of fenestration. https://www.macorners.shop/product-category/tumbler-cup-couple-card/