Caffeate 3,4-dioxygenase (EC 1.13.11.22) is an enzyme that catalyzes the chemical reaction

3,4-dihydroxy-trans-cinnamate + O₂ ${\displaystyle \rightleftharpoons }$ 3-(2-carboxyethenyl)-cis,cis-muconate

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Thus, the two substrates of this enzyme are 3,4-dihydroxy-trans-cinnamate (caffeic acid) and oxygen, whereas its product is 3-(2-carboxyethenyl)-cis,cis-muconate.

This enzyme belongs to the family of oxidoreductases, specifically those acting on single donors with O₂ as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O₂. The systematic name of this enzyme class is 3,4-dihydroxy-trans-cinnamate:oxygen 3,4-oxidoreductase (decyclizing). This enzyme participates in phenylpropanoid biodegradation.