The most common etiologic factor of primary hyperparathyroidism is a solitary parathyroid adenoma that might be located in the mediastinum. The mediastinally located ectopic parathyroid adenomas are often out of reach with a cervical incision and require a thoracic approach. We present a case of 55 year-old female patient with a parathyroid adenoma located anterior to the ascending aorta within the thymus resulting in primary hyperparathyroidism. The patient underwent an extended thymectomy with intraoperative use of a gamma probe as an adjunct to surgical resection which provided the accurate localization of the parathyroid adenoma. Histologic diagnosis confirmed the tumor to be an ectopic parathyroid adenoma. The patient showed an uneventful postoperative period with decreased levels of both calcium and parathyroid hormone. We emphasize that the gamma probe serves as a very useful device to differentiate the ectopic parathyroid adenoma from surrounding tissues for complete surgical excision.