A 42-year-old male patient's posteroanterior chest radiography showed homogenous opacity located at right-sided upper paratracheal region and thoracic computed tomography revealed a mass located at posterior mediastinum, which resulted in no complaint. Positron emission tomography showed a benign lesion with an insufficient I-131 uptake. The mass was resected by thoracotomy. Histopathologic examination revealed papillary carcinoma of ectopic thyroid tissue showing follicular variant. Bilateral subtotal thyroidectomy was performed. No malignancy in thyroid tissue was found in the histopathological examination. This case was presented because of being a rare case with follicular carcinoma in the mediastinal thyroid tissue without any malignancy in the thyroid tissue of the neck.