The use of nivolumab as first-line therapy for unresectable advanced gastric cancer has now become a standard practice, and its efficacy has been established. This is the first report of a patient with advanced gastric cancer who underwent conversion surgery after first-line nivolumab combination chemotherapy. The patient was a 58-year-old woman. Her medical history included hypertension and dyslipidemia. She had advanced gastric cancer with extensive lymph node metastasis in the left supraclavicular fossa and around the abdominal aorta. After confirming the HER2-negative status and the PD-L1 CPS score to be ≥5, nivolumab was administered in combination with chemotherapy. After the treatment, she underwent a total gastrectomy with D2 dissection, combined splenectomy and pancreatic tail resection for adhesions, and para-aortic lymph node sampling as a conversion surgery. There was no obvious cancerous remnant in the resected specimen, and the pathological response was Grade 3. The patient was alive and recurrence-free at 4 months postoperatively.