Background　Emerging　studies　suggest　obesity　may　improve　PD-1/PD-L1　inhibitor　efficacy,　correlating　with　prolonged　survival,　known　as　the　＇obesity　paradox＇.　However,　the　impact　of　this　paradox　and　obesity-related　chronic　inflammation　on　immunotherapy　for　advanced　gastric　cancer　(AGC)　has　not　received　sufficient　research.　Methods　Between　January　2018　and　December　2021,　patients　receiving　neoadjuvant　therapy　were　categorized　into　two　groups:　combined　immunotherapy　(ICIs,　n　=　173)　and　neoadjuvant　chemotherapy　(NAC,　n　=　126).　Visceral　(VATI)　and　subcutaneous　adipose　tissue　index　(SATI)　were　obtained　from　pre-treatment　CT　images.　The　systemic　immune-inflammation　index　(SII)　was　calculated　as　platelet　count　multiplied　by　the　neutrophil-to-lymphocyte　ratio.　Results　The　median　age　of　patients　was　64　years　(IQR　56-69),　with　219　(73.2%)　males　and　80　(26.8%)　females.　In　the　ICIs　group,　the　VATI-High　group　showed　significantly　higher　3-year　overall　survival　(OS)　(p　=　0.010)　and　disease-free　survival　(DFS)　(p　=　0.029).　Similar　results　were　observed　in　the　SATI　analysis　(p　＜　0.05).　Conversely,　OS　(p　=　0.040)　and　DFS　(p　=　0.039)　were　significantly　lower　in　the　SII-High　group.　Both　VATI　and　SATI　were　independent　protective　factors　for　OS　and　DFS,　but　the　effect　disappeared　after　adjustment　for　SII.　SII　was　associated　with　poorer　OS　and　DFS,　even　after　adjustment　for　VATI　and　SATI.　No　significant　differences　were　observed　in　the　analysis　of　the　NAC　group.　Conclusions　Elevated　adiposity　indices　(VATI/SATI)　and　low　SII　correlate　with　survival　benefit　in　ICI-treated　AGC　patients,　and　importantly,　this　paradoxical　survival　benefit　is　dependent　on　SII　status.　In　contrast,　no　such　benefit　is　observed　in　chemotherapy-alone　cohorts.