A rare case of a completely isolated, alimentary tract duplication cyst in a 27-day-old neonate is reported. The duplication cyst was detected on antenatal fetal ultrasound and magnetic resonance (MR) imaging at 27 weeks' gestational age. At surgery, the duplication cyst was in a retroperitoneal site with no apparent communication between the cyst and any portion of the alimentary tract. On histopathological examination, the diagnosis was a gastric duplication cyst. The patient's postoperative course was uneventful. There have been eight cases of completely isolated duplication reported in the literature, of which seven were detected during the prenatal or neonatal period. No previous report in the English literature has described the fetal MR imaging findings of this type of duplication cyst.