BACKGROUND A ventricular septal aneurysm (VSA) is rare and almost always an incidental finding on cardiac imaging. It is rarely an isolated phenomenon and is more commonly associated with other forms of congenital heart disease such a ventricular septal defect (VSD). Differentiating a ventricular septal aneurysm from an aneurysm of the right sinus of Valsalva is crucial as the latter usually has a more aggressive course and may require surgical intervention. Cardiac computed tomography (cardiac CT) or cardiac magnetic resonance imaging (CMR) may help confirm the diagnosis. CASE REPORT We report a case of a 42-year-old obese Japanese man with a past medical history of hyperlipidemia who described occasional effort-related palpitations when climbing stairs over the past few months but no anginal symptoms. Echocardiogram revealed normal left ventricular systolic with a presumed right sinus of Valsalva aneurysm measuring around 1.5 cm. A coronary CTA was obtained to further delineate the aneurysm and revealed normal CT angiographic appearance of a right dominant coronary artery circulation with a small aneurysmal outpouching of the membranous ventricular septum measuring 13×17 mm without any evidence of shunting, along with focal calcification of the medial aspect of the tricuspid annulus. The right sinus of Valsalva appeared normal on coronary CTA. CONCLUSIONS Membranous ventricular septal aneurysm is a rare condition that is almost always an incidental finding on echocardiography and can be mistaken for an aneurysm of the right sinus of Valsalva. Multimodality imaging and high degree of clinical suspicion are needed to accurately diagnose a ventricular septal aneurysm and to achieve favorable outcomes. A VSA usually has a benign course and is rarely a cause of arrythmia, right ventricular outflow obstruction, or valvular insufficiency.