Ectopic decidua (deciduosis) has been discovered in variable organs during pregnancy. Ovarian deciduosis, however, is a less frequent event during pregnancy. Ectopic decidua is a physiological phenomenon of pregnancy and arises from a progesterone-induced metaplasia of subserosal stromal cells. As we experienced a case of 21 weeks gestation who was diagnosed with ovarian deciduosis that was suspicious for ovarian malignant tumor, we present it with a brief review of literature.