An 84-year-old man was referred to our hospital with gross hematuria. Abdominal computed tomography demonstrated a well-circumscribed enhanced mass in the right pelviureteral junction. Retroperitoneoscopic nephroureterectomy was performed because of a clinical diagnosis of renal pelvic carcinoma. Pathologic examination led to a final diagnosis of an adult Wilms tumor arising from the renal pelvis. This is the first report of a Wilms tumor in the renal pelvis of an adult patient.