Congenital omental cysts are rare intra-abdominal pathology, which are difficult to diagnose preoperatively; as such a high index of suspicion is required for accurate preoperative diagnosis. We present a case of congenital omental cyst in a 3-year-old girl who presented with huge abdominal distension. We performed diagnostic examinations including ultrasonography and CT of the abdomen. An omental cyst was diagnosed because of its position and connection to the surrounding tissues. She was operated and cyst was excised completely. Histological examination revealed an omental cyst with endothelial lining and haemorrhagic fluid inside. She had an uneventful recovery and doing well, without recurrence at follow-up of 24 months. Clinicians must rigorously pursue a preoperative diagnosis, as it may prevent a surprise upon laparotomy and result in proper management.