Ectopic parathyroid adenoma is a rare pathology. We present a clinical case of a giant ectopic parathyroid adenoma (PA) in an unusual location, which brought significant diagnostical and therapeutical challenges. The tumour in the mediastinum was found incidentally on chest computed tomography (CT). A biopsy was conducted, and histological examination revealed a well-differentiated neuroendocrine tumour. The tumour was excised via right thoracotomy. The final histological examination revealed a parathyroid adenoma, which was unexpected for our team. After three years of observation, there is no evidence of tumour recurrence.