Introduction: The supernumerary kidney, the rarest urinary tract anomaly, typically presents as an asymptomatic third kidney. Renal cell carcinoma constitutes 90% of kidney cancer cases. Supernumerary kidneys are often incidentally discovered via imaging. Management depends on symptoms, with nephrectomy for nonfunctioning kidneys. Only three cases of supernumerary kidney carcinoma are reported. This report aims to report a case of renal cell carcinoma of the left supernumerary kidney discovered intraoperatively. Case report: A 56-year-old woman presented with abdominal enlargement and left flank pain for three months. A palpable lump was found, and a contrast-enhanced CT scan revealed a solid mass at the upper pole of the left kidney with suspected metastasis. During surgery, a large mass was found, leading to a radical nephrectomy. Postoperative recovery was challenging; the patient was moved to intensive care, but her condition worsened, and she died. The pathological diagnosis was clear cell renal cell carcinoma. Conclusion: The incidence of renal cell carcinoma in the supernumerary left kidney is exceedingly rare. Intraoperative findings were different from those observed preoperatively. Radiological assessment is necessary to understand the blood supply, anatomical variations, and associated problems to prevent unnecessary surgery and the associated risk of complications.