Background: Primary ovarian leiomyosarcoma is an extremely rare mesenchymal tumor that comprises less than 0.1% of all ovarian malignancies. Because of its rarity, the diagnosis, treatment, and pathology of primary ovarian leiomyosarcoma often present challenges to physicians. Heterogeneous solid features on imaging and nonspecific bowel symptoms attributed to its close proximity to the gastrointestinal tract often cause confusion with a possible gastrointestinal tumor. Case presentation: A 67-year-old woman presented with a pelvic mass without any specific symptoms. Preoperative imaging studies were suggestive of a mesenchymal tumor of the gastrointestinal tract, namely a gastrointestinal stromal tumor. Serum tumor markers were all within the normal range. The patient underwent explorative laparotomy, and the mass was proven to originate from the ovary. Intraoperative frozen examination suggested a benign fibroma. Pathological examination finally confirmed that the mass was a primary ovarian leiomyosarcoma. Conclusion: Although rare, the possibility of primary ovarian leiomyosarcoma should be considered when the nature and location of a pelvic mass overlap with those of a gastrointestinal tract tumor.