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Immune checkpoint inhibitors have shown efficacy in various malignancies. However, reports of conversion surgery after nivolumab therapy for unresectable advanced esophagogastric junction(EGJ)cancers remain rare. We present the case of a 69-year-old male diagnosed with EGJ cancer(cT4a[diaphragm], N2[No. 1×1, No. 7×1, and No. 19×1], M0, cStage Ⅲ). Neoadjuvant chemotherapy with docetaxel plus S-1 regimen was administered, resulting in stable disease(SD). During treatment, microsatellite instability-high(MSI-H)status was identified, and nivolumab monotherapy was subsequently initiated per regulatory approval. Although the radiological response remained SD, the tumor showed notable shrinkage, and the conversion surgery achieved R0 resection. The pathological findings revealed a Grade 1b treatment effect. The patient continued adjuvant nivolumab therapy postoperatively and remained disease-free. This case highlights the potential role of perioperative immunotherapy with nivolumab in MSI-H EGJ cancers and the promise of personalized treatment strategies.