Introduction: A schwannoma is an encapsulated nerve sheath tumor originating from differentiated Schwann cells. They are preferentially localized to the head, neck and extremities with a minor occurrence of 1-3% in the retroperitoneal space. Schwannoma symptoms often are asymptomatic due to slow growth but can present with symptoms according to their location. We report a case of a 45-year-old female with a retroperitoneal schwannoma that mimicked an ovarian mass. Case Summary: A 45-year-old female with a ten-year history of a right ovarian cyst presented with lower abdominal pain, early satiety, hematuria, low back pain and constipation. Transabdominal ultrasound showed right ovarian enlargement measuring 8.8 x 6.6 cm, described as heterogenous with loculations. Tumor markers indicated a low risk of malignancy. The patient was taken to the operating room for an exploratory laparotomy that revealed a large, firm posterior pelvic mass palpated in the retroperitoneal space. The cyst wall could not be completely dissected due to proximity to major vessels and nerve roots. The patient was referred to a neighboring academic institution because of the case complexity. She underwent repeat exploratory laparotomy and neuroplasty of the L5, S1 and S2 nerve roots. The L5 nerve root was ultimately sacrificed and the retroperitoneal mass was removed. Final pathology revealed a benign schwannoma. Conclusion: The diagnosis of a retroperitoneal schwannoma is often missed because of its rare occurrence, location, vague symptoms and resemblance to other tumors on varying imaging modalities. Due to low incidence and often delay in diagnosis, current management guidelines are limited and not well documented. Currently, histopathological exam after total surgical resection is the most reliable source for diagnosis. The patient in this case demonstrates the need for improvement in the preoperative evaluation and consideration of non-gynecologic differential diagnoses in the evaluation of a suspected pelvic mass.