Schwannomas are the most common peripheral nerve sheath tumors. Benign schwannomas with malignant transformation are rarely reported. Most common schwannomas occur in the head and neck region. Sciatic schwannomas are rare, as are completely cystic schwannomas. Sciatic nerve schwannomas represent less than 1% of all schwannomas. Benign tumors in the sciatic nerve consist of 60% neurofibromas and 38% schwannomas. In general, a schwannoma induces chronic symptoms. It can be misleading, sometimes mimicking degenerative spinal pathology due to disc herniation. Schwannoma involving the sciatic nerve can be asymptomatic or may present with sciatica or neurological deficits. Most schwannomas are solid or heterogeneous tumors, and completely cystic schwannomas are rare. The differential diagnoses of nondiscogenic sciatica include lumbar disc herniation, tumor, abscess, hematoma, facet syndrome, lumbar instability, sacroiliitis, piriformis syndrome, and sciatic neuritis. We report a rare case of a long completely cystic sciatic schwannoma in the left foraminal L5-S1 zone extending to the left ischial groove with chronic sciatica that was diagnosed radiologically with a combination of conventional MRI and MR neurography and confirmed histopathologically by surgical resection. The patient previously had conservative therapy, but the complaints were not reduced. Nonsurgical therapy is considered the first choice, and surgical therapy is indicated in cases that do not respond to conservative therapy, with recurrent cysts, severe pain, or neurological deficits.