Fibrosing mediastinitis (FM) is a rare benign disorder that is characterized by excessive fibrotic reactions in the mediastinum. FM is associated with various diseases, including Histoplasma capsulatum infection and IgG4-related disease, and may compromise the airways, great vessels, and other mediastinal structures. Chylothorax is not a common manifestation of FM, and there is no standard treatment for FM or chylothorax. Recently, however, somatostatin and octreotide, a somatostatin analogue, were successfully used for the treatment of chylothorax due to various causes, and they are considered as putative therapeutic interventions for chylothorax. Here, we present a 28-year-old Japanese man with chylothorax due to idiopathic FM, who was successfully treated with octreotide. The patient visited our hospital because of dyspnea on exertion. On admission, chest computed tomography revealed pericardial effusion, bilateral pleural effusion, and a mass in the mediastinum. The right pleural effusion appeared chylous, with the triglyceride level of 253 mg/dl. The biopsy specimen from the mediastinal mass showed collagenous fibers and fibroblasts with moderate infiltration of lymphocytes. Neither fungi nor bacteria were cultured from the biopsy specimen. Steroid therapy was not effective. The patient was then treated with subcutaneous octreotide (100 μg three times daily). Five days after starting the treatment, the drained pleural fluid was decreased to ∼150 ml/day from ∼1,000 ml/day. The mediastinal mass decreased in size 2 weeks after the initiation of octreotide treatment. After discharge, the patient has received octreotide treatment for 6 months without serious adverse events. We suggest octreotide as a treatment option for FM.