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. Th is is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.