Abstract Background: The incidence of early-onset esophagogastric cancer (EOEGC), defined as age of diagnosis <50, has increased in recent decades. Compared to average-onset esophagogastric cancer (AOEGC), EOEGC are more likely to be genomically stable, have diffuse histology, and present with more advanced disease. Here, we utilized a large real-world data cohort to characterize EOEGC with molecular and immune signatures. Methods: 5,175 esophagogastric cancer (EGC) samples (<50 yrs, n=530; ≥50 yrs, n=4645) were tested by next generation sequencing (NGS) (592, NextSeq; WES, NovaSeq) and whole transcriptome sequencing (WTS) (NovaSeq) (Caris Life Sciences, Phoenix, AZ). EGC patients with age <50 and ≥50 were classified as EOEGC and AOEGC, respectively. Deficient mismatch repair (dMMR)/microsatellite-instability (MSI) was tested by IHC and NGS. Tumor mutational burden (TMB) totaled somatic mutations per tumor (high>10 mt/MB). Pathway enrichment determined by Gene Set Enrichment Analysis (Broad Institute, Cambridge, MA). Immune cell fractions were calculated by deconvolution of WTS: Quantiseq. Statistical significance was determined using chi-square and Mann-Whitney U test with p-values adjusted for multiple comparisons (q < 0.05). Results: EOEGC had higher frequency of CDH1 (18.9% vs 6.5%) but lower frequency of TP53 (65.9% vs 74.4%) mutations compared to AOEGC (all q<0.05). EOEGC had higher frequency of CCNE1 (6.8% vs 4.4%), MYC (6.1% vs 4.1%) and FGFR2 (3.45% vs 1.87%) copy number alterations and ARHGAP26 (5.67% vs 2.2%) fusion. EOEGC had lower frequency of TMB high (3.8 vs 10.1%) and dMMR/MSI-H (1.3% vs 5.7%) (all p<0.05). EOEGC had pathway enrichment of epithelial mesenchymal transition (EMT) (NES: 1.8, FDR: 0.0) and angiogenesis (NES: 1.3, FDR: 0.24). EOEGC had lower median MAPK pathway activity score (MPAS) (-0.16 vs 0.13, p<0.05). Analysis of inferred immune cell infiltrates showed that EOEGC had increased infiltration of B cells (4.7% vs 3.9%) and M2 macrophages (3.6% vs 3%) but decreased infiltration of M1 macrophages (3.7% vs 4%) (all q<0.05). EOEGC had increased T cell inflamed signature (31.3% vs 26.1%, p<0.05) and increased expression of HAVCR2 (TIM3) but decreased expression of FOXP3 and CD274 (PD-L1) (FC: 0.84-1.09, all p<0.05). There was no difference in LAG3, CTLA4, and IDO1 gene expression. Conclusion: EOEGC has several unique molecular and tumor immune microenvironment features. In this real world cohort, EOEGC has increased frequency of CDH1 mutations, ARHGAP26 fusions, enrichment of EMT and angiogenesis pathways, decreased MAPK pathway activity, decreased frequency of TMB high and dMMR/MSI-H, and a unique immune cell infiltrate with decreased M1 macrophages and increased M2 macrophages. These unique differential characteristics present therapeutic opportunities but also demonstrate the limitations of currently approved therapies in this subset of patients. Citation Format: Lawrence W. Wu, Sachin K. Deshmukh, Sharon Wu, Joanne Xiu, Alex Farrell, Vincent Lam, Emil Lou, Sanjay Goel, Chadi Nabhan, Ryan H. Moy. Comprehensive molecular and immunological characterization of early-onset esophagogastric cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3890.