Ovarian dysgerminomas are malignant tumors that are infrequently associated with pregnancy. These tumors are typically detected in cases of abdominal pain or during routine imaging such as ultrasound or magnetic resonance imaging. However, we report a case of ovarian dysgerminoma in an ongoing pregnancy, discovered as a result of hemoperitoneum. A 33-year-old patient at 23 weeks of gestation presented with hemorrhagic shock and hemoperitoneum, requiring emergency surgery. During the operation, a large tumor measuring 24 × 29 cm was found in the left ovary, which was completely invaded by the mass but showed no adhesions to adjacent organs. The condition was accompanied by abundant ascites (4.5 L) mixed with blood, caused by bleeding from an ovarian vessel on the surface of the tumor. Histological examination confirmed the diagnosis of dysgerminoma. Haemoperitoneum is a rare but severe complication that can lead to discovering ovarian dysgerminoma. Owing to the low malignancy of this tumor, adnexectomy is an effective emergency treatment, offering favorable maternal and fetal outcomes in early-stage cases. However, larger case series are required to assess the prognosis of pregnancy associated with dysgerminoma better.