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Prostate cancer (PCa) is one of the most common cancers in males, and its treatment remains challenging due to the tumor microenvironment (TME) with immunosuppressive properties and limited response to anti-PD-1 therapy. Gut microbiota-derived metabolites have recently emerged as modulators of cancer immunometabolism, however, their role in PCa progression and immunotherapy is poorly understood. Here we found that indole-3-lactic acid (ILA), a metabolite produced by Lactobacillus plantarum, exerted dual anti-tumor effects on PCa cells and the TME. Mechanistically, ILA activates the aryl hydrocarbon receptor (AHR), and the resulting AHR/ARNT heterodimer translocates into the nucleus and binds to the promoter of ASF1B. This heterodimer then recruits the HDAC1/2-NuRD complex to reduce H3K27ac levels and suppress ASF1B expression. ASF1B binds to specific residues of ENO1 via its N-terminal core domain and enhances ENO1 enzymatic activity. ILA-induced downregulation of ASF1B impairs this interaction, reduces ENO1 activity, and suppresses the PI3K/Akt pathway, thereby inhibiting the malignant phenotypes of PCa cells. Concurrently, ILA decreased CXCL8 secretion by inhibiting the PI3K/Akt/NF-κB pathway, enhancing CD8⁺ T cell infiltration and M1 macrophage polarization, thereby remodeling the TME. Additionally, ILA synergized with anti-PD-1 therapy to more effectively suppress tumor growth. These findings reveal a novel mechanism by which gut microbiota-derived metabolites regulate PCa progression and immunometabolism, positioning ILA as a potential therapeutic agent to improve PCa treatment.