We report a case of retroperitoneal teratoma diagnosed prenatally by serial sonographic examinations in the third trimester. A 29-year-old woman was referred for sonographic evaluation at 33 weeks’ gestation because of a fetal intra-abdominal mass. Our initial sonographic image suggested a neuroblastoma. Repeat ultrasound images demonstrated an increase in size of the tumor, while the content of the tumor became predominantly solid with areas of calcification. Teratoma should be considered on detection of any cystic or mixed semisolid mass, especially when calcification is present. The fetus was prenatally diagnosed with retroperitoneal teratoma. After birth at 39 weeks, the tumor was removed and histological analysis revealed an immature retroperitoneal teratoma. Intensive monitoring of the changes in ultrasound images of the tumor should provide ground for a precise antenatal diagnosis.