Primary ovarian pregnancy is a rare type of ectopic pregnancy that has been associated with higher chance of rupture compared to other types. We present a 23-year-old woman with severe lower abdominal pain and 4 weeks amenorrhea. On physical examination, she was clinically unstable with distended abdomen and tenderness on palpation. Bimanual pelvic examination revealed cervical motion tenderness, left adnexal mass, and adnexal tenderness. Human chorionic gonadotropin test was positive, and transabdominal ultrasound showed no gestational sac in uterine cavity with massive fluid collection surrounding the uterus that hinder the identification of the ovaries. The exploratory laparotomy revealed a mass on the left ovary with active bleeding. Wedge resection was conducted, and histopathological examination showed chorionic villi and trophoblastic cells embedded in ovarian stroma, confirming the diagnosis of primary ovarian pregnancy. Differentiating ovarian pregnancy from tubal pregnancy, corpus luteum cyst, or ovarian endometrioma via ultrasound remains challenging. © 2025 Royal Thai College of Obstetricians and Gynaecologists. All rights reserved.