Early detection of ovarian pregnancy (OP) is essential for successful laparoscopic conservative surgery. However, early preoperative ultrasonography-based diagnosis is often difficult when fetal cardiac activity or the yolk sac is absent. The authors report a case of OP diagnosed at eight weeks gestational age in a natural pregnancy. The patient presented with amenorrhea and transient vaginal bleeding, and slight tenderness in the right ovary was noted during vaginal ultrasonography. Furthermore, ultrasonography showed a gestational sac (GS) without fetal cardiac activity or yolk sac, consistent with OP, and an adjacent compressible lutein cyst. The uterus, fallopian tubes, and left ovary were normal, and no cul-de-sac blood or ascites were found. Laparoscopy showed a two-cm mass partially covering the right ovary, which contained an unruptured GS. Subsequently, the mass was removed, and OP was histologically confirmed.