A 28-week-old fetus was detected to have a single left atrial mass in prenatal ultrasound. Postnatal echocardiography showed an aneurysm between the anterior mitral leaflet and aortic valve, to the left of atrioventricular junction and communicating with the left ventricle through a narrow mouth. It probably originated from the mitral-aortic intervalvular fibrous tissue and an inherent weakness at this site might be the cause. Reported cases of pseudoaneurysm of mitral-aortic intervalvular fibrosa and subvalvular ventricular aneurysms seen following infective endocarditis, surgery, or trauma seem to have a similar anatomical background. This case explains the possibility of congenital aneurysm in this location which needs to be considered a differential diagnosis in similar cases.