Maleic acid or cis-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO₂CCH=CHCO₂H. Maleic acid is the cis isomer of butenedioic acid, whereas fumaric acid is the trans isomer. Maleic acid is mainly used as a precursor to fumaric acid, and relative to its parent maleic anhydride, which has many applications.

'Maleic acid' from the French acide maléique altered from acide malique, malic acid, from which it was synthesised. The word 'malic' is derived from Latin mālum, meaning 'apple'.

Maleic acid has a heat of combustion of −1,355 kJ/mol., 22.7 kJ/mol higher than that of fumaric acid. Maleic acid is more soluble in water than fumaric acid. The melting point of maleic acid (135 °C) is also much lower than that of fumaric acid (287 °C). As confirmed by X-ray crystallography, maleic acid is planar. Two normal hydrogen bonds are observed, one intramolecular and one intermolecular. Crystallographic analysis shows that the intramolecular hydrogen bond is little affected in the monopotassium salt.

In industry, maleic acid is derived by hydrolysis of maleic anhydride, the latter being produced by oxidation of benzene or butane.

Maleic acid is an industrial raw material for the production of glyoxylic acid by ozonolysis.

Maleic acid may be used to form acid addition salts with drugs to make them more stable, such as indacaterol maleate.

Maleic acid is also used as an adhesion promoter for different substrates, such as nylon and zinc coated metals e.g galvanized steel, in methyl methacrylate based adhesives.

The major industrial use of maleic acid is its conversion to fumaric acid. This conversion, an isomerization, is catalysed by a variety of reagents, such as mineral acids and thiourea. Again, the large difference in water solubility makes fumaric acid purification easy.