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The immune-response gene 1 (IRG1) plays a key role in anti-pathogen defense, as deletion of Irg1 in mice causes severe defects in response to bacterial and viral infection, and decreased survival1, 2. IRG1 transcription is rapidly induced by pathogen infection and inflammatory conditions primarily in cells of myeloid lineage3. IRG1 encodes a mitochondrial metabolic enzyme, aconitate decarboxylase 1 (ACOD1), that catalyzes the decarboxylation of cis-aconitate to produce the anti-inflammatory metabolite itaconic acid (ITA)4. Several molecular processes are affected by ITA, including succinate dehydrogenase (SDH) inhibition5, resulting in succinate accumulation and metabolic reprogramming6, 7, and alkylation of protein cysteine residues, inducing the electrophilic stress response mediated by NRF2 and IκBζ8, 9 and impairing aerobic glycolysis10. However, the mechanisms by which ITA exerts its profound anti-inflammatory effect still remains to be fully elucidated. Here, we show that ITA is a potent inhibitor of the TET family DNA dioxygenases, which catalyze the conversion of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC) during the process of active DNA demethylation. ITA binds to the same site of α-ketoglutarate (α-KG) in TET2, inhibiting its catalytic activity. Lipopolysaccharides (LPS) treatment, which induces Irg1 expression and ITA accumulation, inhibits Tet activity in macrophages. Transcriptome analysis reveals TET2 is a major target of ITA in suppressing LPS-induced genes, including those regulated by NF-κB and STAT signaling pathways. In vivo, ITA decreases 5hmC, reduces LPS-induced acute pulmonary edema and lung and liver injury, and protects mice against lethal endotoxaemia in a manner that is dependent on the catalytic activity of Tet2. Our study thus identifies ITA as an immune modulatory metabolite that selectively inhibits TET enzymes to dampen the inflammatory response.