Osteomalacia is a metabolic bone disorder characterized by impaired mineralization of bone matrix. Symptoms of osteomalacia can be confused with other conditions such as spondyloarthropathy, polymyalgia rheumatica, polymyositis, and fibromyalgia. In this case, we report a patient with axial osteomalacia who developed low back pain, morning stiffness, and "grade 3 sacroiliitis" in pelvis X-ray, leading to the misdiagnosis as seronegative spondyloarthropathy. Serum biochemical studies revealed low serum phosphorus, low 25-hydroxy vitamin D3, normal calcium, elevated parathyroid hormone, and alkaline phosphatase levels. Her symptoms were relieved with vitamin D and calcium therapy. The diagnosis of osteomalacia should be considered in case of sacroiliitis and spondylitis.