In this case report, a novel N-acetylgalactosaminyltransferase 3 homozygous mutation (c.782 G>A; p.R261Q) associated with hyperphosphatemic familial tumoral calcinosis/hyperostosis-hyperphosphatemia syndrome is described. The patient had elbow, pelvis, and lower limb pain and a hard mass in the hip and olecranon regions. Increased levels of inorganic phosphorus (Pi) and C-reactive protein were observed. After treating the patient with conventional drugs, we tested denosumab, which reduced but did not normalize the Pi.