Spontaneous ovarian heterotopic pregnancy is extremely rare. Ovarian ectopic pregnancy frequently resembles hemorrhagic corpus luteum cyst, but possibly mimics ovarian malignant tumor due to inhomogeneous echogenic appearance in some instances. Emergency laparotomy was performed for a seven-week spontaneous ovarian heterotopic pregnancy, because the ovarian cystic tumor exhibited a solid part on ultrasonography, therefore malignancy could not be ruled out. Postoperative course was uneventful, and the intrauterine fetus grew without complications, resulting in spontaneous vaginal delivery at 39 weeks of gestation. The possibility of an ovarian heterotopic pregnancy should be suspected, when an ovarian tumor is detected during pregnancy. These findings help physicians to diagnose ovarian heterotopic pregnancy or isolated ovarian ectopic pregnancy, to reduce maternal morbidity and mortality, and provide satisfactorily ongoing intrauterine gestation in heterotopic pregnancy.