Schwannoma (neurilemmoma) is a peripheral nerve sheath tumor and commonly occurs singularly on the head, neck, and trunk. Giant schwannoma is rarely located on the retroperitoneum and pelvic cavity. The majority of symptoms caused by the tumor are due to the effect of its mass. Surgical resection is enough to treat the tumor. Schwannoma is reported usually as benign, and despite incomplete resection of the tumor, the risk of recurrence and metastasis is low. A schwannoma on the retroperitoneum that was preoperatively misdiagnosed as a malignant adnexal mass in a 60-year-old menopausal woman is presented, with a brief review of the literature.