Background: Ovarian ectopic pregnancy is a rare form of non-tubal ectopic pregnancy. It can rupture before the end of the first trimester, causing hemoperitoneum, and present with signs and symptoms similar to other commoner abdominal emergencies or the pregnancy can continue intraperitoneally. Therefore, they are not often diagnosed preoperatively. Ultrasound can assist in diagnosis of ovarian ectopic pregnancy but the findings could be ambiguous or inconclusive. We present a case of ruptured ovarian ectopic pregnancy at the second trimester causing massive hemoperitoneum that was suspected as an intrabdominal malignancy co-existing with intrabdominal pregnancy. Case presentation: She was a 34 year-old Nigerian unbooked G4P3+0, (3 alive), who presented to the labor ward on 21st January, 2021 with a complaint of a 6-week history of abdominal pain and swelling. Pain was insidious in onset, generalized, non-colicky, non-radiating, constant, no known aggravating or relieving factor, but it was of moderate intensity. She had amenorrhea with a positive serum pregnancy test without prior early ultrasound. At presentation, initial abdominopelvic ultrasound revealed intra-uterine viable pregnancy but repeat ultrasound done showed a left adnexal ectopic gestation and an echo-rich intraperitoneal fluid collection. Laparotomy was done and ovarian pregnancy was accurately diagnosed intra-operatively. Tissue samples from the ovary confirmed normal products of conception, namely chorionic villi, trophoblastic cells and ovarian stroma at histology. Conclusion: Despite advances in imaging techniques, the diagnosis of ovarian ectopic gestation is still very difficult. When premenopausal women present with amenorrhea, generalized non-colicky abdominal pain and swelling in combination with ambiguous findings of pregnancy on ultrasound in the absence of trauma, differential diagnoses should include ruptured ovarian pregnancy. Obstetricians should maintain a high index of suspicion to forestall delayed diagnosis and the potential maternal morbidity and mortality. However, the need for high-index of suspicion should be for any ectopic, not just ovarian pregnancy.