Solitary extramedullary plasmacytoma is an uncommon neoplasm and occurs most frequently in the upper respiratory tract. Herein, we reported a solitary extramedullary plasmacytoma in the retroperitoneum. A 28-year-old man presented with obstructive jaundice and a retroperitoneal tumor. Ultrasound-guided biopsy confirmed that the lesion was a plasma cell neoplasm. A detailed investigation showed that no other sites were involved. The tumor got a moderate reduction following local irradiation, and a complete remission was achieved after 12 courses of adjuvant chemotherapy. Therefore, the possibility of a solitary extramedullary plasmacytoma should be considered in the differential diagnosis of obstructive jaundice without a history of multiple myeloma.