Pericardial cysts are rare mediastinal abnormalities, which are usually congenital but may also be acquired after cardiothoracic surgery. Pericardial cysts are most commonly located at the cardiophrenic angle or, rarely, in the posterior or anterior superior mediastinum. The majority of pericardial cysts are asymptomatic and are found incidentally on imaging studies. Our case was an 18-year-old female with a diagnosis of ostium secundum atrial septal defect (ASD). She was planned for surgical closure of ASD. On the opening of the pericardium, a pericardial cyst of around 3.0×2.0-cm size, filled with clear fluid, was found in the aorta–pulmonary artery groove. Marsupialization of the cyst was done followed by routine direct suture closure of ASD. Postoperative recovery was uneventful, and the patient was discharged on the third postoperative day. Histopathology of tissue confirmed our diagnosis. Our aim to present this case report is to show an incidental finding of the pericardial cyst at a rare location and to discuss its perioperative implications.