By reason of a new case of an ovarian mucinous borderline tumor (BOT) in a pre-menarche girl, a research of current literature was implemented. Low-grade malignant epithelial tumors are extremely rare in young children and, as far as we know, only a few case reports exist. The patients presented with vomiting, pain, and a swollen lower abdomen. Pre-operative diagnosis primarily consists of imaging techniques. At Stage Ia, the tumor is confined to the ovary without penetration of the capsule, no malignant ascites or peritoneal implants. Treatment consists of removal of the tumor combined with concurrent salpingo oophorectomy, appendectomy, omentectomy, and peritoneal lavage. Although the treatment recommendations are not uniform, basically, preservation of fertility is the main objective. The prognosis is very good, but recurrence is possible even after 10 years.