A primary ovarian leiomyosarcoma is extremely rare. Moreover, there is no established treatment modality other than surgery, and the prognosis is extremely poor. We report a case of a primary ovarian leiomyosarcoma. The patient was a gravida 4, para 2, 73-year-old female. She first presented with chief complaints of constipation and a pelvic mass. A physical examination revealed a solid tumor in the pelvic cavity. Which was about the size of an infant's head and had an irregular shape. The patient was suspected of having a subserosal myoma of the uterus or an ovarian tumor, and she was subjected to a laparotomy. A solid tumor about the size of an infant's head was found in the left uterine adnexa, and an intraoperative rapid pathological diagnosis of an ovarian leiomyosarcoma was made. A total abdominal hysterectomy, a bilateral salpingo-oophorectomy, a pelvic lymphadenectomy, and an omentectomy were then performed. The final pathological diagnosis confirmed a left primary ovarian leiomyosarcoma. After a series of discussions with the patient's family about her therapeutic plan, we decided to let the patient be followed-up without adjuvant therapy. Forty-two months after the surgery, the patient accumulated massive amounts of pleural effusion and ascites, with extensive organ metastasis. She received terminal care and soon died.