<p>The prognosis for unresectable advanced gastric cancer remains poor. Chemotherapy is the primary treatment, and the recent introduction of immune checkpoint inhibitors (ICIs) has improved outcomes. Despite their benefits, ICIs can cause severe immune-related adverse events, necessitating careful administration. Conversion surgery, which involves radical resection after a positive response to chemotherapy, has shown better outcomes than chemotherapy alone. A 74-year-old woman presented with abdominal distension, and was diagnosed with advanced gastric cancer invading the transverse colon and causing obstruction. Radical resection was deemed difficult; hence, a bypass for colon obstruction was performed. Intraoperative assessments revealed transverse colon invasion, peritoneal dissemination, and positive results in lavage cytological examinations, confirming stage 4 gastric cancer. Postoperative chemotherapy with S-1, oxaliplatin, and nivolumab was initiated. She developed severe immune-related liver injury, responded to steroids, and resumed chemotherapy without nivolumab after recovery. After three cycles, significant tumor reduction and disappearance of peritoneal dissemination were observed. Conversion surgery, including right hemicolectomy and distal gastrectomy, was performed to achieve R0 resection. The patient was discharged on the ninth postoperative day and underwent adjuvant chemotherapy with S-1. She remained recurrence-free 18 months after surgery. This case demonstrated successful downstaging using chemotherapy with ICIs and subsequent radical resection via conversion surgery. Thus, conversion surgery is a viable option for multidisciplinary gastric cancer treatment and may see increased application with aggressive chemotherapy plus ICI regimens for managing metastatic or unresectable gastric cancer.</p>