Background: Extramedullary plasmacytoma is an immunoproliferative monoclonal disease of the B-cell line that originates from malignant transformed plasma cells. It’s a rare variant of plasma cell tumor involving organs outside the bone marrow, found in 13% of cases. Urinary bladder involvement remains a location rarely reported in the literature. Herein, we report a case of bladder extramedullary plasmacytoma revealing multiple myeloma. Case report: A 49-year-old man presented to the urology department with an acute obstructive renal failure revealed by renal colic and dysuria gradually evolving over 2 months. The renal ultrasound and the pelvic abdominal CT scan revealed a bilateral ureter hydronephrosis with a locally advanced bladder tumor. The patient underwent surgical resection of the tumor and the histopathology study of surgical specimen showed an infiltration of the urothelial mucosa by abnormal plasma cells. The bone marrow aspirate established the presence of 59% of abnormal plasma cells and rouleaux formation of red blood cells, serum protein electrophoresis showed a monoclonal peak in the area of gamma globulin, and the conventional radiography of the axial skeleton objectifies lytic spine lesions. The diagnosis of a multiple myeloma associated to an extramedullary bladder location was retained. The outcome was unfavorable and the patient died of severe pulmonary embolism before initiating any therapy. Conclusion: Secondary extramedullary plasmacytoma is a marker of poor prognosis in both newly diagnosed and relapsed multiple myeloma patients. Bladder location remains a rare entity, the clinical presentation is nonspecific and management constitutes a therapeutic challenge even in the era of new agents.