Lymphangioma is a congenital abnormality of the lymphatic system detected primarily in early childhood. There are rare reports of mediastinal lymphangioma in older adults. We hereby report on a 66-year-old female patient who underwent kidney transplantation 20 years previously and who developed pathologically confirmed solitary mediastinal lymphangioma 1 year ago. Chest radiography showed a mediastinal nodule, which was not observed 2 year previously, therefore she was referred to the pulmonary division. She had no symptoms, and chest computed tomography demonstrated a 25-mm, well-defined, low-density nodule located at the anterior mediastinum. The size of the nodule had increased from 25 mm to 34 mm 1 year later, and it was completely resected via video-assisted thoracic surgery. The histological diagnosis was cystic lymphangioma. Therefore, we recommend that clinicians consider cystic lymphangioma as a possible diagnosis even in older patients with a mediastinal cystic mass that shows progressive enlargement.