Objective. Immunotherapy has proven effective in treating advanced gastric cancer (AGC), yet its benefits are limited to a subset of patients. Our aim is to swiftly identify prognostic biomarkers using cytokines to improve the precision of clinical guidance and decision‐making for PD‐1 inhibitor‐based cancer immunotherapy in AGC. Materials and Methods. The retrospective study compared 36 patients with AGC who received combined anti‐PD‐1 immunotherapy and chemotherapy (immunochemotherapy) with a control group of 20 patients who received chemotherapy alone. The concentrations of TNF‐α, IL‐1β, IL‐2R, IL‐6, IL‐8, IL‐10, and IL‐17 in the serum were assessed using chemiluminescence immunoassay at three distinct time intervals following the commencement of immunochemotherapy. Results. When compared to controls, patients undergoing immunochemotherapy demonstrated a generalized rise in cytokine levels after the start of treatment. However, patients who benefited from immunochemotherapy showed a decrease in IL‐6 or IL‐8 concentrations throughout treatment (with varied trends observed for IL‐1β, IL‐2R, IL‐10, IL‐17, and TNF‐α) was evident in patients benefiting from immunochemotherapy but not in those who did not benefit. Among these markers, the combination of IL‐6, IL‐8, and CEA showed optimal predictive performance for short‐term efficacy of immunochemotherapy in AGC patients. Conclusion. Reductions in IL‐6/IL‐8 levels observed during immunochemotherapy correlated with increased responsiveness to treatment effectiveness. These easily accessible blood‐based biomarkers are predictive and rapid and may play a crucial role in identifying individuals likely to derive benefits from PD‐1 blockade immunotherapy.