Paget's disease of the bone may predispose the development of malignant bone tumors such as osteosarcoma. Giant cell tumor (GCT) as a consequence of Paget's disease is rare. Bone GCT is characterized by rapid growth, the destruction of bone, extension to the surrounding soft tissue and abnormal bone turnover caused by an abnormality of the receptor activator of nuclear factor-kappa B (RANK)-RANK ligand (RANKL) pathway. Denosumab is a RANK-RANKL inhibitor, which is used to treat osteoporosis and bone GCT. In the current study, a 60-year-old male presented with severe pain located between the right thigh and the knee. The patient could not bear weight on the affected leg. The patient had suffered from Paget's disease for 15 years. The complications from Paget's disease included degenerative hip disease, for which the patient underwent a right total hip replacement. A right periacetabular lesion was identified and confirmed as Paget's disease-induced GCT by needle biopsy. A positron emission tomography (PET) scan revealed significant tumor metabolic activity. Subsequent to obtaining informed consent, the patient started treatment with denosumab. A total of 2.5 months after starting denosumab, a PET scan showed no residual pathological uptake at the site of the previously identified large PET avid tumor. After 1 year, the patient exhibited a satisfactory clinical improvement. In conclusion, treatment with denosumab markedly reduced the size of the hemi-pelvic GCT and led to a complete metabolic response.