Beckwith–Wiedemann syndrome (BWS) is a common overgrowth syndrome that involves abdominal wall defects, macroglossia, and gigantism. It is sometimes complicated by placental tumor and polyhydramnios. We report a case of BWS, prenatally diagnosed with ultrasonography. A large and well–circumscribed tumor also existed on the fetal surface of the placenta, which was histologically diagnosed as chorangioma after delivery. Polyhydramnios was obvious and the fetal heart enlarged progressively during pregnancy. Because the biophysical profiling score dropped to 4 points at 33 weeks of gestation, we carried out cesarean section. By epigenetic analysis, H19–differentially methylated region hypermethylation was observed in the placental tumor, normal placental tissue, and cord blood mononuclear cells. This is the first report of BWS with placental tumor due to H19–differentially methylated region hypermethylation.