The prognosis of patients with peritoneal metastasis from gastric cancer is extremely poor. Although chemotherapy combined with immunotherapy has achieved promising efficacy in the first-line treatment of advanced gastric cancer, patients with peritoneal metastasis benefit less from this regimen. Hyperthermic intraperitoneal chemotherapy (HIPEC) represents a novel treatment option, which maintains the high concentration of drugs in the abdominal cavity, and improve the anti-tumor efficacy of chemotherapy drugs through the thermo-thermal effect. The purpose of this study is to investigate the efficacy and safety of HIPEC and systemic chemotherapy combined with sintilimab in the first-line treatment of advanced gastric cancer and gastroesophageal junction adenocarcinoma with peritoneal metastasis.