Introduction: Parasitic leiomyomas are extremely rare benign smooth muscle tumors that develop independently of the uterus. They may arise spontaneously from pedunculated subserosal fibroids that detach and establish an independent blood supply from surrounding structures, or they may develop iatrogenically following laparoscopic morcellation. Owing to their nonspecific clinical and radiologic features, they are often misdiagnosed preoperatively. Surgical excision with histopathological verification remains the standard of care. Case report: We present the case of a 51-year-old postmenopausal woman with progressive abdominal distension and a sensation of pelvic pressure. Imaging revealed a large heterogeneous abdominopelvic mass measuring 30 × 20 cm, compressing adjacent bowel loops and major vessels. The uterus was myomatous but of normal size, and both ovaries appeared unremarkable. The patient underwent open surgical resection of the mass with total hysterectomy, bilateral adnexectomy, and omentectomy. Intraoperatively, a giant tumor connected to the uterine fundus by a thin pedicle was identified. Histopathological examination confirmed a benign leiomyoma without atypia or necrosis and a Ki-67 proliferation index below 1%. The postoperative course was uneventful, and the patient remained asymptomatic three months after surgery. Conclusion: Primary parasitic leiomyomas are exceedingly uncommon, particularly in patients without a history of prior gynecologic surgery. They should be considered in the differential diagnosis of large abdominopelvic masses. This case emphasizes the importance of comprehensive diagnostic evaluation and highlights that parasitic leiomyomas can attain remarkable size while maintaining benign histological features.