Giant hydronephrosis owing to ureteropelvic junction obstruction is a rare condition characterized by the accumulation of more than 1000 ml of urine in the pyelocaliceal system. It could mimic the other benign cystic kidney disease or other causes of abdominal mass in radiologic images. We reported a 16-year-old female who presented with three months of abdominal pain and gradual abdominal mass ingrowth. Abdominal computed tomography scan showed a giant left cystic mass favored hydronephrosis secondary to ureteropelvic junction obstruction. The patient underwent a left nephrectomy, and more than 12 litters of turbid urine were suctioned from the affected kidney. In conclusion, giant hydronephrosis is an infrequent entity and should be considered in the differential diagnosis of large cystic abdominal masses. The treatment is determined by the underlying cause and the visual appeal of the affected kidney.