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Reprogramming of lipid metabolism and cyclic GMP‒AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling is associated with cancer development. However, whether and how fatty acid metabolism regulates the cGAS‒STING pathway in colorectal cancer (CRC) remains to be elucidated. In this study, we found that short-chain acyl-CoA dehydrogenase (ACADS) is aberrantly deficient in CRC cells and is associated with cancer progression in human patients. We further revealed that ablation of ACADS promoted CRC progression by orchestrating the cGAS‒STING signaling-dependent immunosuppressive tumor microenvironment (TME) in mouse xenografts and AOM/DSS-induced CRC models. Mechanistically, ACADS deficiency suppressed cGAS‒STING signaling by inhibiting mtDNA leakage in a nonmetabolic manner. ACADS binds to and inhibits mitochondrial DNMT1 (mito-DNMT1)-dependent mtDNA methylation, thereby stabilizing mtDNA and inhibiting its leakage. Genetic and pharmacological modulation of mito-DNMT1 restored ACADS-regulated mtDNA leakage, cGAS‒STING signaling, and CRC progression. Importantly, strong correlations between ACADS, mito-DNMT1, and STING signaling and the immune TME were found in patients with CRC. Furthermore, we screened and identified an old drug, hypericin, as an ACADS-binding compound that upregulates ACADS expression. Hypericin treatment can mimic ACADS overexpression-regulated pathways, ultimately improving the immune TME and suppressing CRC growth. These findings highlight a previously undiscovered ACADS/mito-DNMT1 complex that links fatty acid metabolism reprogramming to mtDNA methylation and cGAS‒STING signaling-dependent antitumor immunity.