Hyperparathyroidism-jaw tumour syndrome (HPT-JT) is a rare, familial autosomal dominant condition characterised by parathyroid tumours associated with ossifying fibromas of the jaw, uterine and renal lesions. It is caused by a germline mutation in the CDC73 tumour suppressor gene which encodes parafibromin, a protein with anti-proliferative activity. CDC73 mutations are primarily associated with HPT-JT, familial isolated hyperparathyroidism, and sporadic parathyroid carcinoma. Patients with HPT-JT have up to 23% life-time risk of parathyroid cancer. We present a case of a 30-year-old man with HPT-JT who presented to our unit with long-standing severe primary hyperparathyroidism (PHPT), associated with an unusual cement-osseous dysplasia of the maxilla and mandible. Parathyroid imaging studies initially identified a solitary adenoma. He underwent bilateral exploration and a single parathyroidectomy, weighing 6.73grams, however the surgery failed with persistent hyperparathyroidism post-operatively. Further investigations, including selective venous sampling, revealed an ectopic adenoma in the right submandibular region. Cure was achieved following resection of this tumour. Much remains unclear regarding HPT-JT. Incomplete penetrance, variable expression and unclear genotype-phenotype association make the diagnosis challenging. Phenotypic variability may also lead to underdiagnosis. The optimal surgical approach to PHPT in HPT-JT remains unclear, and there are also no well-established guidelines for surveillance. Reporting of cases is essential to improve clinician awareness, better recognise the heterogeneity of this syndrome, and guide future research.