Ectopic thyroid tissue (ETT) is a rare entity and a challenging differential diagnosis. This is a report of a case of a mediastinal mass that was found to be an ectopic mediastinal thyroid tissue, in a 77-year-old woman who was admitted to our hospital for breast cancer management. The mediastinal mass was identified in the postsurgical computed tomography (CT) scan of the chest and was suspected as mediastinal lymph node metastasis. A CT-guided percutaneous transthoracic punch biopsy (CT-TPB) proved to be an adequate diagnostic tool to exclude malignancy and provide a definite diagnosis of the mediastinal mass. We find that CT-guided punch biopsy as a useful diagnostic alternative enabling histopathological specimens to be obtained from mediastinal masses and lymph nodes suspected of malignancy.