A 74-year-old woman with cT4aN2M0, cStage ⅢB gastric cancer underwent neoadjuvant chemotherapy comprising 2 courses of S-1 plus cisplatin, and the clinical response was determined as non-CR/non-PD according to RECIST ver 1.1. Although distal gastrectomy with D2 lymphadenectomy was planned, the tumor was considered as unresectable with peritoneal metastases during laparotomy. After the subsequent chemotherapy with 1 course of capecitabine plus cisplatin, tumor bleeding, and obstruction due to rapid tumor progression occurred. We performed palliative distal gastrectomy; however, the patient died 17 days after gastrectomy. A comprehensive genomic analysis using cancer-gene panel identified the tumor as a microsatellite instability-high(MSI-H). Recently post hoc analysis of the large-scale clinical trials showed no clinical benefit of perioperative chemotherapy in MSI-H gastric cancer. MSI status has a potential to optimize the perioperative treatment strategy in gastric cancer.