Duplication cyst occupying the retro- and intra-peritoneal space is a rare condition. We describe a case of duplication cyst in a 13-year-old girl who presented with abdominal pain, vomiting, and a lower abdominal mass. Plain abdominal X-ray films revealed local ileus over the lower abdomen. Ultrasonography revealed 2 double-layered cystic masses over the lower abdomen with a suspicious communicating tract. Mild hydropelvis of the right kidney was also noted. Abdominal computed tomography revealed 2 cystic lesions. One was located at the pelvic cavity just above the urinary bladder and the other was in the left retroperitoneal space. Laparotomy revealed a dumbbell-shaped intra-retroperitoneal duplication cyst with a small communicating tract. The cyst was excised without disturbing bowel continuity and the vascular supply. The patient was doing well at 1-year follow-up.