Introduction: X-linked hypophosphataemia (XLH) is the most common inherited cause of renal phosphate wasting, typically diagnosed in childhood but occasionally identified in adulthood. Case report: We report a case of a 45-year-old woman with long-standing lower limb deformities, short stature, and diffuse joint pain. Laboratory evaluation revealed hypophosphataemia due to isolated renal phosphate wasting. Fibroblast growth factor 23 levels were markedly elevated. Imaging showed skeletal deformities and enthesopathic changes consistent with chronic XLH. Genetic testing identified a pathogenic PHEX variant (c.1645+1G>A), confirming the diagnosis. Treatment with oral phosphate and calcitriol resulted in biochemical improvement. During follow-up, the patient developed symmetrical erosive polyarthritis of the hands and wrists, consistent with seronegative rheumatoid arthritis superimposed on the underlying metabolic disorder. Discussion: This case highlights the diagnostic challenge of XLH in adults, where musculoskeletal manifestations can closely mimic inflammatory arthritis. Recognising XLH as the underlying cause of chronic osteomalacia is essential to avoid inappropriate treatment and ensure timely management of coexisting conditions. New-onset erosive arthritis should prompt consideration of a concurrent inflammatory condition rather than attribution to the underlying metabolic disorder.