Retroperitoneal undifferentiated sarcoma is a soft tissue sarcoma that is highly positive for vimentin but has no reactions to various cell-specific markers in immunohistochemical study. This tumor has never been reported in a hemodialysis patient with chronically immunosuppressed uremic state. We report a case of retroperitoneal undifferentiated sarcoma in a 50-year-old uremic man who presented with intermittent left flank dull pain and painless hematuria. MRI revealed a huge left perirenal mass, and complete resection was achieved. The pathological examination of the tumor cells revealed poorly-differentiated spindle or oval-shaped sarcoma cells, and the immunohistochemical study excluded the diagnosis of rhabdomyosarcoma, leimyosarcoma, liposarcoma, angiomyolipoma, sarcomatoid renal cell carcinoma, and sarcomatoid transitional cell carcinoma. This indicated that the sarcoma did not show any differentiation. The patient had an uneventful postoperative course and is currently well 3 years after operation. We review the clinical picture of this tumor in a hemodialysis patient, elucidate the ”undifferentiated” histopathological characteristics by immunohistochemical study and point out the possible connection between sarcoma formation and immunosuppressive status from uremia.