A 63-year-old woman was referred to our hospital because screening had detected an anterior mediastinal tumor with a right aortic arch. She underwent typical total thymectomy via a median sternotomy but developed left recurrent laryngeal nerve (RLN) palsy postoperatively. The pathology examination revealed that the tumor was a stage I thymoma. This is the first report of a thymoma with a right aortic arch. The left RLN goes around the left ductus arteriosus, which connects the origin of the left subclavian artery to the left pulmonary artery. The RLN was likely to be injured in the neighborhood of the left ductus arteriosus when the tumor and thymus were dissected over the pulmonary artery. During operations for an anterior mediastinal tumor with a right aortic arch, we should be attentive to the location of the tumor, the left ductus arteriosus, and the left RLN.