Itaconic acid is an organic compound with the formula CH₂=C(CO₂H)CH₂CO₂H. With two carboxyl groups, it is classified as a dicarboxylic acid. It is a non-toxic white solid that is soluble in water and several organic solvents. It plays several roles in biology.

Upon heating, itaconic acid converts to its anhydride.

As a dicarboxylic acid, itaconic acid has two pKa's. At pH levels above 7, itaconic acid exists as its double negatively charged form, termed itaconate.

As an α,β-unsaturated carbonyl compound, itaconic acid is a good Michael acceptor. Thus, nucleophiles add across the C=C bond.

This reaction is the means by which the fire retarding chemical 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide can be incorporated into polymers.

In 1836, Samuel Baup discovered itaconic acid as a by-product in a dry distillation of citric acid. In the late 1920s, itaconic acid was isolated from a fungus in the Aspergillus genus of fungi The dry distillation forms itaconic anhydride, which then is hydrolyzed. Since the 1960s, however, it has been produced commercially by fermenting glucose, molasses, or another abundant carbon sources by a fungus such as Aspergillus itaconicus, Aspergillus terreus, or Ustilago maydis have also been investigated. One generally accepted route by which fungi make itaconate is through the tricarboxylic acid cycle pathway. This pathway forms cis-aconitate which is converted to itaconate by cis-aconitate-decarboxylase. Animal cells also make itaconate by an enzyme-catalyzed reaction from cis-aconitate, an intermediate metabolite in the tricarboxylic acid cycle, (i.e., TCA cycle). The itaconate-producing reaction is stimulated when the TCA cycle is suppressed.

Ustilago maydis makes itaconic acid from trans-aconitate, catalyzed by aconitate delta-isomerase. The trans-aconitate product is decarboxylated to itaconate by trans-aconitate decarboxylase (i.e., TAD1, an enzyme found in Ustilago maydis) Itaconate has also been obtained by fermenting the fungi Yarrowia lipolytica with glucose, various species of Candida fungi with glucose, Ustilago vetiveriae fungus with glycerol, and various species of Aspergillus niger fungi with glucose, sorbitol, or sorbitol plus xylose mixture. Fermenting Escherichia coli bacteria with glucose, xylose, glycerol, or starch and Corynebacterium glutamicum bacteria with glucose or urea also affords itaconic acid. Ustilago maydis has, however, been genetically engineered to increase its itaconic acid production,

In the 1930s itaconate was shown to have bactericidal actions. In 2011, Strelko et al. reported that itaconate was produced by two mammalian immortalized cell lines, cultured mouse VM-M3 brain tumor cells and RAW 264.7 mouse macrophages, and by macrophages isolated from mice. This group also showed that stimulation of mouse macrophages with the bacterial toxin, lipopolysaccharide (i.e., LPS, also termed endotoxin), increased their production and secretion of itaconate. In 2013, Michelucci et al. revealed the biosynthesis pathway that makes itaconate in mammals. These publications were followed by numerous others focused on the biology of itaconate and certain itaconate-like compounds as regulars of various cellular responses in animals and possibly humans.