The patient was a 34-year-old woman who had been routinely monitored after receiving a childhood diagnosis of partial anomalous pulmonary venous connection, but unilaterally discontinued follow-up examinations after the age of 18. At 33 years of age, she was admitted to our hospital after a physical examination revealed an abnormal shadow on a chest X-ray. Transthoracic echocardiography detected an atrial septal defect (ASD), and contrast-enhanced computed tomography showed that the right lower pulmonary vein drained to the inferior vena cava. The patient was diagnosed with scimitar syndrome with ASD. Cardiac catheterization showed a pulmonary/systemic flow ratio (Qp/Qs) of 2.48 and a left-to-right shunt rate of 59.7%. Surgical treatment was deemed to be indicated. The right lower pulmonary vein was anastomosed to the anterolateral wall of the right atrium, and an intra-atrial baffle repair was performed from the orifice within the right atrium to the left atrium through the existing ASD using untreated fresh autologous pericardium. Two years after the operation, good blood flow was maintained within the baffle with no stenosis at the anastomotic site. This report describes a rare case of scimitar syndrome with ASD in an adult woman, and provides a review of the existing literature.