We report a case of rare and aggressive ovarian carcinosarcoma with a germline pathogenic BRCA2 variant. A patient with a history of breast cancer who developed an inflammatory ovarian tumor with peritonitis carcinomatosis involving the appendix suffered from cachexia. Following three cycles of weekly paclitaxel and carboplatin chemotherapy, emergency surgery was required owing to sepsis. Bilateral salpingo-oophorectomy, total hysterectomy, appendectomy, and small intestine adhesiolysis were performed. Histologically, the tumor comprised an admixture of carcinomatous and sarcomatous components, with involvement of the appendix, which had caused perforation and abscess formation. The final diagnosis was ovarian carcinosarcoma with a germline pathogenic BRCA2 variant, c.658_659del (p.Val220fs). The patient responded completely to adjuvant chemotherapy. A combination of chemotherapy and surgery might be beneficial to patients with ovarian carcinosarcoma and germline pathogenic BRCA2 variants with a poor general condition. This is the first report of ovarian carcinosarcoma with a germline pathogenic BRCA2 variant that responded favorably to chemotherapy.