Sinus of Valsalva aneurysm is a rare congenital or acquired cardiac structural abnormality. The sinus of Valsalva aneurysm refers to the local development defect of the aortic sinus wall. Under aortic hypertension, the sinus wall becomes thinner and expands outward. Its clinical manifestations range from asymptomatic to severe life-threatening complications caused by compression or rupture of important surrounding structures. A few cases of aortic aneurysm sinus with arrhythmias have been reported, including complete heart block. Here, we present the case of a 50-year-old male patient with recurrent syncope due to a third-degree atrioventricular block. We found a right sinus of Valsalva aneurysm that ruptured into the basal segment of the interventricular septum and formed a basal septal dissection on echocardiography, which could have caused a third-degree atrioventricular block. This case report highlights the importance of finding secondary etiologies in the sudden or transient onset of third-degree atrioventricular block and using echocardiography to evaluate patients with heart block.