Background: The increased PD-L1 expression evaluated by combined positive score (CPS) is associated with improved efficacy of immunotherapy in GE cancers. The impact of tumor molecular alterations on PD-L1 expression is still not well-studied. We aimed to characterize specific molecular features of tumors with different CPS levels in GE cancers. Methods: 2,707 GE tumors [1,662 gastric/GE junction adenocarcinoma (GA), 856 esophageal adenocarcinoma (EA), 75 esophageal squamous (ES) and 114 GE unspecified] collected between 2000.8 and 2019.7 were analyzed using NextGen DNA sequencing (NGS), immunohistochemistry (IHC) and fragment analysis (FA) (Caris Life Sciences, Phoenix, AZ). Tumor mutation burden (TMB) was calculated based on somatic nonsynonymous missense mutations. dMMR/MSI status was evaluated by a combination of IHC, FA and NGS. PD-L1 expression measured by IHC (22c3) was evaluated by CPS scores. Molecular alterations were compared in three groups (CPS ≥ 10, H; CPS = 1~9, M; CPS = 0, L) using Fisher-Exact or Chi-square and adjusted for multiple comparison by Benjamini-Hochberg. Significance was determined by adjusted (adj) p < .05. Results: Overall, CPS-H, M, and L were seen in 18% (n = 494), 28% (n = 765) and 53% (n = 1,448) of GE tumors respectively. CPS-H was the most prevalent in ES (43%) followed by GA (19%) and lowest in EA (14%). Overall, TMB was similar between CPS-L and M, but was significantly increased in H (average TMB = 8.4 vs. 8.6 vs. 11 mt/MB, adj p < .0001); the effect was seen in EA and GA, but not in ES. An overall significant association between MSI/dMMR status and PD-L1 expression levels was seen (2%, 3.2% and 12% in CPS-L, M and H, adj p < .05) in GE tumors; the significance was seen in GA, but not in EA or ES. Amplifications of PD-L1 (H: 1.5%, M: 0.1% and L: 0) and PD-L2 (H: 1.1%, M: 0.1%, L: 0) were the highest in CPS-H, while ASPSCR1 (H: 0, M: 0, L: 1%) and TNFRSF14 (H: 0, M: 0.4, L: 2%) were the lowest (adj p < .01). Genes involved in epigenetic modification (top 5: ARID1A, ASXL1, BCL9, BCOR, CREBBP), MAPK (KRAS, MAP2K1) and mismatch repair (MLH1, MSH6) had the highest mutation rates in CPS-H, compared to M and L (p < .0001). In contrast, CDH1 had higher mutation rates in CPS-L (12%), compared to M and H (5% and 5%) (p < .0001). Conclusions: This is the largest study to investigate the distinct molecular landscape of pts with different PD-L1 expression levels in GE cancers. Our data may provide novel insights for pt selection (e.g. pts with gene mutations involved in epigenetic modification) and the development of rational combination immunotherapy (e.g. drugs targeting MAPK pathway).