A 55-year-old woman presented with intermittent fatigue and abdominal pain. Ultrasound and CT scan showed a large heterogeneous mass in the retroperitoneum, suggestive of malignancy. F-FDG PET/CT was performed for staging. PET/CT images showed a hypermetabolic retroperitoneal mass. Endoscopy revealed a mass infiltrating the duodenum. Eventually, the mass was pathologically proved to be malignant melanoma. Further extensive clinical, radiological, and endoscopic investigations proved the retroperitoneum to be the primary site.