A germ cell tumor that involves the retroperitoneal space. The disease is retroperitoneal germ cell neoplasm (Monarch Disease Ontology entry MONDO_0003685). Also known as: germ cell neoplasm of retroperitoneum, germ cell neoplasm of the retroperitoneum, germ cell tumor of retroperitoneum, germ cell tumor of the retroperitoneum, germ cell tumour of retroperitoneum, germ cell tumour of the retroperitoneum, retroperitoneal germ cell tumor, retroperitoneal germ cell tumour.