Torsion of fetal ovarian cyst is a rare congenital disease and is difficult to diagnose due to its variable sonographic images. We report this case, which was prenatally diagnosed as torsion of a fetal ovarian cyst by clinicl manifestation of fetal tachycardia, and fetal ultrasonographic morphological changes such as multiple septa and internal echoes of the ovarian cyst at 34 and 36 weeks of gestation.