Abstract Promoter elements play important roles in isoform and cell type–specific expression. We surveyed the epigenomic promoter landscape of gastric adenocarcinoma, analyzing 110 chromatin profiles (H3K4me3, H3K4me1, H3K27ac) of primary gastric cancers, gastric cancer lines, and nonmalignant gastric tissues. We identified nearly 2,000 promoter alterations (somatic promoters), many deregulated in various epithelial malignancies and mapping frequently to alternative promoters within the same gene, generating potential pro-oncogenic isoforms (RASA3). Somatic promoter–associated N-terminal peptides displaying relative depletion in tumors exhibited high-affinity MHC binding predictions and elicited potent T-cell responses in vitro, suggesting a mechanism for reducing tumor antigenicity. In multiple patient cohorts, gastric cancers with high somatic promoter usage also displayed reduced T-cell cytolytic marker expression. Somatic promoters are enriched in PRC2 occupancy, display sensitivity to EZH2 therapeutic inhibition, and are associated with novel cancer-associated transcripts. By generating tumor-specific isoforms and decreasing tumor antigenicity, epigenomic promoter alterations may thus drive intrinsic tumorigenesis and also allow nascent cancers to evade host immunity. Significance: We apply epigenomic profiling to demarcate the promoter landscape of gastric cancer. Many tumor-specific promoters activate different promoters in the same gene, some generating pro-oncogenic isoforms. Tumor-specific promoters also reduce tumor antigenicity by causing relative depletion of immunogenic peptides, contributing to cancer immunoediting and allowing tumors to evade host immune attack. Cancer Discov; 7(6); 630–51. ©2017 AACR. This article is highlighted in the In This Issue feature, p. 539