A mediastinal Mullerian cyst was initially reported as a new category of congenital cyst by Hattori, et al. in 2005. We treated a 53-year-old female referred to us with a posterior mediastinal tumor found at the Th5 prevertebral level by chest-computed tomography during a medical check-up. She had a history of mediastinal teratoma, which was removed at the age of 35. Chest magnetic resonance imaging revealed homogenous, high-intensity signals in T2-weighted images. The lesion was resected using a thoracoscopic procedure, and histologic and immunohistochemical staining revealed a ciliated cyst of Mullerian origin. The newly established mediastinal Mullerian cyst should be included in the differential diagnosis of posterior mediastinal cysts.