We herein report a case of an intrathoracic low-grade fibromyxoid sarcoma arising from the chest wall and associated with massive pleural effusion. A 70-year-old man presented with a persistent cough. A chest computed tomography scan revealed a large mass in the right pleural cavity with massive pleural effusion. No malignant cells were recognized in the pleural effusion by thoracentesis. A malignant soft tissue tumor was suspected, and surgery was performed. The tumor arose from the posterior chest wall and was resected with the connected chest wall. The definitive diagnosis was a low grade fibromyxoid sarcoma. Because the posterior margin of the chest wall was microscopically tumor positive, postoperative irradiation was performed. The patient has now been followed up for 30 months with no evidence of recurrence.