When obstructive lesions from the uterus or ovaries are suspected, patients with hydronephrosis are usually referred to a gynecologist. Here, a case of suspected endometriosis-related hydroureteronephrosis is reported. A 43-year-old woman with hydronephrosis was found to have a left distal periureteral tumor on the computerized tomography scan. Before the operation, the hydroureteronephrosis was suspected caused by the obstruction of ureter, related with ureteral endometriosis; however, the postoperative pathology revealed the diagnosis of retroperitoneal well-differentiated liposarcoma. When female patients have hydronephrosis, gynecologic causes should be considered. Both benign and malignant causes are needed to include when making differential diagnosis. Therefore, robot-assisted surgery is a feasible option because of its lower morbidity rate and more precise dissection of soft tissue than laparotomy in both benign and malignant retroperitoneal tumors.