Postpartum osteoporosis (PO) is a rare condition characterized by low bone mineral density (BMD) and an increased risk of vertebral fragility fracture. We encountered a 34-year-old woman who developed back pain 1 week after delivery. Magnetic resonance imaging of the lumbar spine revealed three vertebral compression fractures. Pretreatment BMD evaluation by dual-energy X-ray absorptiometry revealed a low T-score and Z-score (−2.0 and −2.0, respectively; BMD, 0.876 g/cm2) in the affected region of the spine. The patient was diagnosed with PO and treated with subcutaneous injection of denosumab 60 mg (Prolia; Amgen, Inc., Thousand Oaks, CA, USA) every 6 months. After two treatments, the BMD had significantly increased and the back pain was improved; the patient therefore decided to terminate the treatment. Two months later, her back pain worsened and BMD decreased as measured by dual-energy X-ray absorptiometry examination of the lumbar spine. Therefore, the patient resumed treatment with denosumab, and the BMD of the lumbar spine increased after another two treatments. Therefore, we consider denosumab to be promising in the management of PO with respect to increased BMD and decreased pain.