Peripheral nerve sheath tumor (PNST) involving the sacrum occurs rarely, and malignant PNST is even rarer. These tumors are usually of considerable size at the time of diagnosis. We report on a 65-year-old woman with a rare malignant PNST in the sacral region who had complained of severe right hip pain and urinary and fecal incontinence for years. The lumbar magnetic resonance imagines showed a large pelvic mass. The tumor mass was completely removed through the posterior approach, and the pathological report showed this to be a highly aggressive malignant PNST, which usually has a poor prognosis. We discuss the therapeutic modalities and prognosis.