Giant hydronephrosis is mostly caused by ureteropelvic junction obstruction (UPJO). Giant hydronephrosis with concurrent malignancy is less common clinically and is easily misdiagnosed. We report a 77-year-old male who presented with sudden onset progressive abdominal distension in the last month. Abdominal computed tomography showed a right severe hydronephrotic kidney with loss of parenchymal thinning. Cystoscopy showed a 1x1cm papillary lesion protruding from the right ureteric orifice. He underwent a right radical nephroureterectomy with bladder cuff excision with lymph node dissection. Histopathology showed low-grade urothelial carcinoma of the ureter and incidental pleomorphic rhabdomyosarcoma in the right kidney. The patient refused chemotherapy and died 6 months later due to lung metastasis. Incidental pathologic finding of renal rhabdomyosarcoma in adults with giant hydronephrosis and urothelial carcinoma is a rare occurrence with diverse clinical presentations, prognoses, and outcomes.