We present a case involving a giant hemangioma of the fetal neck, prenatal diagnosis of which was a teratoma. A 32-year-old pregnant woman was referred to our hospital at 31 weeks’ gestation owing to a giant solid mass of the fetal neck and excessive amniotic fluid. The mass seemed to be occupying the neck almost entirely, extending to the nasopharyngeal cavity, the mandible, the surface of the left orbit and the left shoulder. Based on sonographic and magnetic resonance imaging (MRI) findings, diagnosis of a giant teratoma was made. Cesarean section was performed at 35.5 weeks’ gestation, and a female infant weighing 2826 g was delivered. Purpurae were observed on the neck of the infant, and the tumor turned out to be a hemangioma. Postnatal MRI findings, in which the tumor's signal intensity differed from that of the prenatal findings, were quite compatible with the diagnosis of a typical hemangioma. Laser and corticosteroid treatment successfully decreased the volume of the mass. Although it may not always be possible to make a prenatal differential diagnosis between a hemangioma and a teratoma in the neck of the fetus, serial ultrasound and MRI examination are mandatory to evaluate the prognosis and to plan suitable treatment. Moreover, possible postnatal changes to the tumor characteristics have to be taken into consideration when evaluating the findings of prenatal diagnostic imaging.