A mediastinal Müllerian cyst is composed of heterotopic cystic Müllerian tissue resembling structures of the fallopian tube. We herein report a case of a mediastinal Müllerian cyst discovered during a medical check-up of a 41-year-old woman. She had no symptoms but had been diagnosed with hyperprolactinemia at the age of 24 years, for which she received hormonal therapy for 3 years. Chest computed tomography demonstrated a 3-cm-diameter cystic tumor in front of the Th10 vertebra. Thoracic surgery was performed to remove the tumor. The tumor was a monolocular cyst covered with a thin capsule, and pathological examination showed a thin-walled cyst lined by ciliated or non-ciliated columnar cells resembling tubal epithelium. Immunohistochemical analysis showed positive expression for paired box gene 8 and estrogen and progesterone receptors. The pathological diagnosis was a Müllerian cyst. A Müllerian cyst should be always be considered in patients with a mediastinal cyst.