Low-grade appendiceal mucinous neoplasm (LAMN) is a rare epithelial malignancy of the appendix. If it perforates the abdominal cavity, it can cause a serious clinical syndrome called pseudomyxoma peritonei. In the present case, we laparoscopically removed a LAMN encountered during risk-reducing salpingo-oophorectomy (RRSO). The patient was a 53-year-old woman who was diagnosed with hereditary breast and ovarian cancer syndrome. RRSO was planned, and magnetic resonance imaging revealed a large cystic tumor in the right lower abdomen. We expected an ovarian cyst; however, it was a primary tumor of the appendix. Partial cecal resection was performed laparoscopically by a surgical oncologist. The pathological diagnosis was LAMN. Gynecologists may encounter this disease incidentally. Mucinous appendiceal neoplasm (MAN) may be encountered during RRSO. If a right lower abdominal mass is found near a normal ovary preoperatively, gynecologists should consider MAN as well as paraovarian cyst.