Crotonic acid ((2E)-but-2-enoic acid) is a short-chain unsaturated carboxylic acid described by the formula CH₃CH=CHCO₂H. The name crotonic acid was given because it was erroneously thought to be a saponification product of croton oil. It crystallizes as colorless needles from hot water. With a cis-alkene, Isocrotonic acid is an isomer of crotonic acid. Crotonic acid is soluble in water and many organic solvents. Its odor is similar to that of butyric acid.

Crotonic acid produced industrially by oxidation of crotonaldehyde:

A number of other methods exist, including the Knoevenagel condensation of acetaldehyde with malonic acid in pyridine:

The alkaline hydrolysis of allyl cyanide followed by the intramolecular rearrangement of the double bond:

Furthermore, it is formed during the distillation of 3-hydroxybutyric acid:

Crotonic acid crystallizes in the monoclinic crystal system in the space group P21/a (space group 14, position 3) with the lattice parameters a = 971 pm, b = 690 pm, c = 775 pm and β = 104.0°. The unit cell contains four formula units.

Crotonic acid converts into butyric acid by hydrogenation or by reduction with zinc and sulfuric acid.

Upon treatment with chlorine or bromine, crotonic acid converts to 2,3-dihalobutyric acids: