The valves of right horn of systemic venous sinus are prominent structures within the right atrium during early embryonic period. Involution of these structures may not be complete, resulting in a spectrum of anatomical presentations such as cyanosis. A full-term male neonate referred to our hospital for precise evaluation of severe cyanosis on the first day of life. Echocardiographic examination revealed right-to-left interatrial shunting through patent foramen ovale due to prominent eustachian valve with normal estimated right heart pressures from peak tricuspid regurgitation velocity. He was maintained with supplemental oxygen and a PDE-5 inhibitor (sildenafil) and subsequently improvement in oxygen saturation was achieved. The patient was discharged after 2 weeks of treatment. In conclusion; the embryologic remmants of the sinus venosus rarely may lead to right-to-left shunting resulting in severe cyanosis. Pulmonary vasodilators such as sildenafil may improve oxygen saturation in these patients even in case of normal right heart pressures.