Introduction and importanceSquamous cell carcinoma (SCC) arising from mature cystic teratoma of the ovary (MCT-SCC) has a poor prognosis at advanced stages. Although the relationship between homologous recombination deficiency (HRD) and platinum-based chemotherapy sensitivity or poly (ADP ribose) polymerase (PARP) inhibitor efficacy in epithelial ovarian cancer has been demonstrated in clinical trials, the significance of HRD status in MCT-SCC has not previously been described.Case presentationA 73-year-old woman underwent emergency laparotomy due to ovarian tumor rupture. The ovarian tumor was strongly adherent to the surrounding pelvic organs and could not be completely resected. The postoperative diagnosis was stage IIIB MCT-SCC (pT3bNXM0) of the left ovary. After surgery, we conducted the myChoice CDx. The genomic instability (GI) score of 87 was remarkably high, and there was noBRCA1/2pathogenic mutation. After six courses of combination therapy with paclitaxel and carboplatin, the residual tumors had shrunk by 73 %. We performed interval debulking surgery (IDS), and the residual tumors were completely resected. Subsequently, the patient underwent two courses of the combination of paclitaxel, carboplatin, and bevacizumab, followed by maintenance therapy with olaparib and bevacizumab. Twelve months after IDS, no recurrence has been observed.Clinical discussionThe present case suggests that there are some HRD cases among MCT-SCC patients and that IDS and maintenance therapy with PARP inhibitors may be effective in such cases, as in epithelial ovarian cancer.ConclusionAlthough the frequency of HRD-positive status in MCT-SCC remains unknown, HRD testing may provide appropriate treatment options for advanced MCT-SCC.