Left atrial appendage aneurysms are usually congenital and are very infrequent anomalies of the heart. They are very rarely diagnosed during childhood, with most cases symptomatic between the 2nd and 4th decades of life. Diagnosis is vitally important due to potential life-threatening complications. Surgical excision is the treatment of choice. Surgery reduces the risks of cardiac arrest, respiratory distress, arrhythmia, heart failure, thromboembolism, or rupture. We report the case of a 3-year-old boy with incidental diagnosis of a giant aneurysm of the left atrial appendage that was confirmed with magnetic resonance imaging and treated with surgery.