Although most parathyroid adenomas are located in the neck, approximately 25% of them are found deep in the mediastinum. These ectopic adenomas are one of the major causes for failure of bilateral neck exploration because it is almost impossible to reach deep mediastinal tissues from a standard neck incision unless sternotomy is performed. So, although preoperative localization methods are not routinely recommended for the surgical planning of primary hyperparathyroidism, whenever ectopic parathyroid tissues is suspected, preoperative localization can guide the surgery and prevent surgical failure. In this report, we present a case with the diagnosis of primary hyperparathyroidism in whom preoperative Tc-99m-MIBI double-phase scintigraphy, performed for an ongoing research project, revealed an unexpected hyperfunctioning parathyroid nodule in the mediastinum. Subsequent computed tomography and magnetic resonance imaging confirmed an ectopic mediastinal parathyroid adenoma. The surgical approach was modified so that a median sternotomy as well as bilateral neck exploration was performed. A large intrathymic parathyroid adenoma was identified and excised through the sternotomy. In this case, Tc-99m-MIBI scintigraphy successfully localized an unexpected hyperfunctioning mediastinal parathyroid adenoma and played a key role in the surgical planning for the patient. Although not recommended for routine preoperative evaluation, scintigraphy can be useful in the preoperative localization of ectopic parathyroid adenomas, and it may make it possible for the surgeon to modify the surgical approach in selected cases.