Introduction and importance: Giant hydronephrosis is defined as the accumulation of more than 1 l of fluid in the kidney’s collecting system. Its presentation can mimic other conditions including ovarian tumor. Herein, the authors report a case of giant hydronephrosis caused by urolithiasis that mimics the presentation of an ovarian tumor. The authors also highlight the challenges in diagnosing this rare entity and the management options available. Case presentation: The authors present a case of a 65-year-old P5A0 woman with an abdominal tumor of 1-year duration that progressively increased in size. She also complained of mild left flank pain since a year ago. Ultrasonography revealed a large cystic mass in the lower to middle abdomen. Ovarian tumor was suspected and laparotomy was performed. Surgical exploration revealed a left giant hydronephrosis with normal gynecological organs. The postoperative course was uneventful, and she was discharged in satisfactory condition. Clinical discussion: Giant hydronephrosis should always be considered as a differential diagnosis when presented with a large abdominal cystic lesion. Conclusion: Routine screening for bilateral kidneys during gynecological ultrasonography will help detect giant hydronephrosis and prevent unplanned surgical interventions.