Background <p>Endometriosis is an estrogen-dependent benign inflammatory disorder characterized by the implantation of active endometrial tissue (glands or stroma) outside the uterus, with pelvic adhesions as a core hallmark. Omental endometriosis is an extremely rare subtype of extra-pelvic endometriosis (accounting for &lt; 5% of all extra-pelvic endometriosis cases), and its concurrent presentation with massive hemorrhagic ascites and de novo pelvic encapsulated effusion without predisposing factors (e.g., pelvic surgery, pelvic inflammatory disease) remains scarcely reported in existing clinical literature.</p> Case presentation <p>A 45-year-old premenopausal woman (gravida 4, para 2) presented with the chief complaint of increased menstrual flow and prolonged menstruation, without classic endometriosis symptoms such as dysmenorrhea or chronic pelvic pain and no history of abdominal/pelvic surgery or pelvic inflammatory disease. Examination revealed a significant amount of hemorrhagic ascites, pelvic encapsulated effusion, multiple omental nodules on abdominal CT, mildly elevated serum CA125 (85.51 IU/ml) with normal HE4 (61.22 pmol/L), and no malignant cells in ascitic cytology. The cause of hemorrhagic ascites was not definitively diagnosed preoperatively due to overlapping clinical and imaging features with malignant tumors; postoperative pathological and immunohistochemical findings confirmed omental endometriosis.</p> Conclusion <p>Omental endometriosis should be considered in premenopausal women presenting with unexplained hemorrhagic ascites and pelvic encapsulated effusion, even in the absence of classic endometriosis symptoms, prior pelvic surgery, or malignant cytology—particularly when serum CA125 is elevated but HE4 remains normal. This case broadens the recognized clinical spectrum of extra-pelvic endometriosis and highlights the value of comprehensive preoperative evaluation combined with individualized, fertility-sparing surgical management.</p>