Low back pain (LBP) is a common presentation in patients with multiple myeloma. However, LBP that is increased or unrelieved by rest may indicate for malignancy and a patient's young age may mislead and delay diagnosis. Here, we presented a 43 year-old-female who complained of low back pain with radiation to left buttock. The symptom got worsen at night and not improved after took nonsteroidal anti-inflammatory medication. X-ray of lumbarsacral spine revealed ostelytic lesion over sacral bone. Magnetic resonance (MR) imaging studies of lumbarsacral spine showed an infiltrative tumor involving the left-sided sacral bone. Accordingly, left sacral laminectomy with removal of tumor was performed and pathological result revealed plasmacytoma. In addition, bone marrow biopsy demonstrated increased plasma cells with 35% of all nucleated cells. The final diagnosis was multiple myeloma.