A 62-year-old woman, who had been diagnosed with a posterior mediastinal tumor five years ago, was introduced to our department because of enlargement of the tumor. The tumor showed a dumbbell-shaped progression and invasion from the thoracic cavity into the spinal canal through the 3rd intervertebral foramen. On positron emission tomography-computed tomography (PET-CT), increased uptake of FDG was observed in the tumor and bilateral cervical lymph nodes. Based on the image findings, we diagnosed that the tumor was neurogenic and that resection would be necessary due to the possibility of spinal cord compression. Complete removal of the tumor was achieved in conjunction with orthopedic surgeons. Both the pathology of the resected tumor and the biopsy specimens from the mandibular lymph nodes revealed IgG4-positive plasma cells. Additionally, elevated serum IgG4 levels were also observed. Thus, the final diagnosis of the tumor was IgG4-related disease. After the operation, the patient was carefully followed up without any medication. To the best of our knowledge, this paper is the first report of a mediastinal dumbbell-shaped tumor originating from an IgG4-related disease.