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Hypoxia-inducible factor 1 (HIF-1) orchestrates the transcriptional regulation of thousands of genes involved in breast cancer (BC) progression. Here, we identified protein phosphatase 2A (PP2A) methylesterase 1 (PPME1) as a critical HIF-1 target gene that drives oncogenic signaling under hypoxic conditions. In BC cells, HIF-1-dependent PPME1 expression caused inhibition of the PP2A catalytic subunit (PP2Ac), thereby diminishing PP2A activity, which led to AKT activation, phosphorylation of β-catenin, and its nuclear translocation. Nuclear β-catenin cooperates with HIF-1 to promote BC stem cell specification by activating transcription of the NANOG and KLF4 genes, which encode pluripotency factors, and to drive immune evasion by activating transcription of VEGFA, which recruits and polarizes immunosuppressive tumor-associated macrophages and ISG20, which represses STAT1/IRF1-dependent expression of CXCL10, thereby impairing CD8+ T cell recruitment. In vivo, PPME1 knockdown altered the tumor immune microenvironment, enhanced antitumor immunity, and synergized with anti-CTLA-4 immunotherapy to enable complete tumor eradication. These findings establish PPME1 as a critical regulator linking hypoxia signaling, stemness, and immune evasion and highlight its potential as a BC therapeutic target in combination with immune checkpoint blockade.