Schwannoma in retroperitoneal region is quite rare. In this report, we presented such a rare case of a large size schwannoma in a 23-year-old man admitted with a complaint of mild pain in his right lower abdomen. The abdominal ultrasonography detected a round-shaped heterogeneous hypoechoic mass with a few blood flow signals at the retroperitoneal area inferior to the right kidney. The contrast-enhanced ultrasound demonstrated a well-demarcated hypoechoic mass with minor vascularization. Computed tomography (CT) diagnosed the retroperitoneal mass as a malignant neoplasm. After surgical resection, histopathologic examination revealed that the mass was a benign cellular schwannoma. Besides the rare case of schwannoma in retroperitoneal region, we believe that multimodal sonographic patterns are conducive to the preoperative diagnosis of retroperitoneal neurilemmoma.