We report a case of dermatomyositis (DM) associated with invasive thymoma in a 22-year-old woman who was admitted to our hospital complaining of dyspnea which required ventilation support. The reddened elevated scaly eruptions were prominent over the extensor surfaces. Chest X-ray and computed tomography showed mediastinal masses, which were diagnosed as mixed type thymoma. Muscle and skin biopsy specimens were compatible with DM. She was treated with methylprednisolone pulse therapy followed by extended removal of the anterior mediastinal tumor and subsequent radiotherapy. She has had a good clinical course without recurrence of thymoma or DM for more than 3 years. The role of thymoma in the development of DM is discussed.