Background: Hemoperitoneum in pregnancy requires urgent evaluation. While spontaneous intraperitoneal bleeding is rare, ectopic endometrial tissue is a frequent cause of this event. Case: A 38-year-old woman with a history of endometriosis presented at 26 weeks gestation with 1 week of vague abdominal pain. Vital signs were within normal limits, and physical exam was notable for left-sided abdominal tenderness. Imaging demonstrated simple free fluid in her pelvis, concern for a uterine fundal defect and an adjacent hematoma. Exploratory laparotomy revealed hemoperitoneum secondary to highly vascularized stage 4 endometriosis. After classical cesarean delivery, a supracervical hysterectomy with bilateral oophorectomy was performed due to ongoing global pelvic hemorrhage. Conclusion: Consider endometriosis as a cause of spontaneous hemoperitoneum in pregnancy. Obstetricians should be prepared for significant maternal morbidity when encountering such pathology.