If a testicular cancer patient has a mass in the retroperitoneum, a metastasis is often the first suspicion, probably leading to improper diagnosis and overtreatment. Here we report a case of retroperitoneal schwannoma mimicking metastatic seminoma. A 29-year-old man, who had a history of seminoma, presented with a single retroperitoneal mass suspected to be a metastasis. Because the patient refused radiotherapy, 3 cycles of cisplatin, etoposide, and bleomycin were offered. Post-chemotherapy computed tomography scan revealed persistence of the retroperitoneal mass, with no change in tumor size or characteristics. Subsequently, retroperitoneal lymph node dissection was performed. The dissected tissue contained negative lymph nodes but a single mass in the attached fat. Pathology revealed retroperitoneal schwannoma, which was confirmed by immunohistochemistry. Thus, clinicians should be aware of retroperitoneal schwannoma and its distinction from metastatic seminoma to avoid misdiagnosis and ensure proper treatment.