BACKGROUND: Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer and lacks definite treatment targets. Tumor immune microenvironment (TIME) heterogeneity has a profound impact on the immunotherapy response. Tumors with non-inflamed TIME derive limited benefit from immunotherapy. However, what drives the formation of the non-inflamed TIME in TNBC remains unclear. METHODS: Using our multiomics database of TNBC, we conducted an analysis to explore the key genomic events driving the formation of the non-inflamed TIME in TNBC. In vitro and in vivo studies further revealed potential mechanisms and the efficacy of combination treatment with immunotherapy. RESULTS: With transcriptomic and genomic data, we systematically analyzed the TIME of TNBC and revealed that the classical basal-like subtype of TNBC consisted of two distinct microenvironment phenotypes, defined as the 'inflamed' and 'non-inflamed' subtypes. We performed further screening and demonstrated that MYC amplification and overexpression led to low immune infiltration and cytolytic activity in TIME. Mechanistically, MYC bound to DNMT1 promoter and activated DNMT1 transcription in TNBC cells, thus suppressing the Cyclic GMP-AMP synthase (cGAS)-STING pathway via an epigenetic regulatory way. In MYC-overexpressing TNBC, decitabine, an Food and Drug Administration (FDA)-approved DNA methyltransferase inhibitor, converted tumors from non-inflamed to inflamed tumors by enhancing T cell infiltration. Furthermore, the combination of decitabine with programmed cell death protein 1 (PD-1) inhibitor reversed T cell exhaustion and improved T cell function in mouse models, which elicited potent antitumor activity in MYC-overexpressing TNBC. CONCLUSIONS: Our work elucidates that the classic oncogene MYC induces immune evasion by repressing innate immunity. Furthermore, we provide a rationale for combining DNA methyltransferase inhibition with immunotherapy for the treatment of MYC-overexpressing TNBC.