The aim of the study was to evaluate the cost-effectiveness of nivolumab plus chemotherapy as first-line treatment for patients with advanced gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma from the perspective of Chinese and US society.To conduct the analysis, a state-transitioned Markov model, which included three mutually exclusive health states (progression-free survival (PFS), progressive disease (PD), and death), was developed. Cycle length was set at 3 weeks and lifetime horizon was set at 10 years. Costs, quality-adjusted life years (QALYs), and incremental cost-effectiveness ratio (ICER) were calculated in the analysis. Willingness-to-pay (WTP) thresholds in the model were set at $37,653.00/QALY in China and $100,000.00/QALY in the US, respectively. Meanwhile, one-way sensitivity analyses and probabilistic sensitivity analyses were conducted to investigate the robustness of the model.Over a lifetime horizon, the ICERs of nivolumab plus chemotherapy versus chemotherapy alone were $430,185.04/QALY and $944,089.78/QALY in China and the US, respectively. Cost of nivolumab and utility for the PFS state had the most significant impact on ICERs both in the US and China based on the results of the one-way sensitivity analyses. In the probabilistic sensitivity analyses, the proportions of nivolumab plus chemotherapy being cost-effective compared with chemotherapy alone were 0%.In conclusion, nivolumab plus chemotherapy is unlikely to be a cost-effective treatment option compared with chemotherapy alone in the first-line setting of advanced gastric, GEJ, or esophageal adenocarcinoma.© 2023. BioMed Central Ltd., part of Springer Nature.