Background: There are few biomarkers to guide treatment for gastric and gastroesophageal junction adenocarcinoma (G+GEJ). The systemic inflammatory response, which is reflected by circulating serum cytokine levels, of G+GEJ patients is associated with survival. In this study, we evaluated the relationship of circulating serum cytokine levels with overall survival (OS) and pathologic tumor regression grade (TRG) in G+GEJ patients. Methods: A retrospective review of a prospectively maintained biobank was performed to identify patients diagnosed with G+GEJ from 2016-2022. Pre-treatment serum was collected at the time of diagnosis. For patients who received neoadjuvant therapy, an additional serum sample was collected immediately prior to surgical resection. A multivariable Cox proportional hazards model was used to assess the association of cytokine concentration with OS. In patients who received neoadjuvant therapy, we assessed whether the change in IL-6 (ΔIL-6) after therapy was associated with TRG. Results: We first measured the pre-treatment levels of 17 different cytokines in a 67-patient cohort to determine if certain circulating cytokines were prognostic for OS. In a multivariable analysis adjusted for clinical stage and the expression level of the other cytokines, the only circulating cytokine level that was independently associated with OS was IL-6 (HR: 1.06 per 1 pg/ml increase, 95% CI: 1.04-1.08). We then analyzed the serum IL-6 level in an additional 134 patients, resulting in a final cohort of 201 patients. Serum IL-6 level ranged from 0.3-196.7 pg/ml. In a multivariable analysis adjusted for age, sex, clinical stage, pathologic differentiation, Lauren classification, H. pylori status and neutrophil-to-lymphocyte ratio, IL-6 level was independently associated with OS (HR: 1.012 per 1 pg/ml increase, 95% CI: 1.006-1.019). Patients in the top tercile of pre-treatment IL-6 level had worse median OS (10.6 months) compared to patients in the intermediate (17.4 months) and bottom tercile (35.8 months, p < 0.0001). Among patients who received neoadjuvant therapy (n = 50), ΔIL-6 ≤ 0 had a sensitivity of 80% and specificity of 80% for predicting complete or near-complete pathologic tumor regression (TRG 1). Conclusions: Pre-treatment serum level of IL-6 is a promising prognostic biomarker for patients with G+GEJ. ΔIL-6 may predict response to neoadjuvant therapy.