The majority of middle mediastinal cysts are benign diseases. However, some cases of middle mediastinal cysts are revealed to be malignant tumor. We herein report a case of a middle mediastinal cyst, which proved to be a right paratracheal lymph node metastasis from the thyroid papillary carcinoma. The patient was a 67-year-old lady who had underwent thyroidectomy and regional lymph node dissection for papillary thyroid carcinoma 6 years before. Follow-up computed tomography revealed a growing cyst at the right-side of the intrathoracic trachea. Metastasis from thyroid cancer was suspected from her history. The right upper mediastinal lymphadenectomy through thoracoscopy was performed. Pathologically, the cyst was diagnosed as a metastasis from thyroid cancer, showing cystic change.