Hormonal therapy is an important treatment option for estrogen receptor (ER)-positive patients with advanced ovarian cancer. Although ER overexpression has been previously used as an indicator for hormonal therapy, the clinical outcomes of advanced ovarian cancer patients receiving hormonal therapy remain unsatisfactory. Additional biomarkers for screening patients are needed to improve its efficacy. In this study, we reported a metastatic ovarian cancer case with estrogen receptor 1 (ESR1) gene amplification and protein overexpression, which showed sustained partial response to hormonal therapy, including letrozole and tamoxifen, and displayed an overall survival of 47 months. The response to the therapy was evaluated by imageological examinations, cancer antigen-125 (CA-125) tests, and circulating tumor DNA (ctDNA) sequencing using capture-based hybrid next-generation sequencing. Our clinical data suggested that ESR1 amplification might be a potential predictor of response to hormonal therapy in ovarian cancer. The combination of tumor detection techniques including imaging, CA-125 and ctDNA would enable confirmation of tumor response with high confidence.