Caffeate 3,4-dioxygenase () is an enzyme that catalyzes the chemical reaction
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 O<sub>2</sub> as oxidant and incorporation of two atoms of oxygen into the substrate (oxygenases). The oxygen incorporated need not be derived from O<sub>2</sub>. The systematic name of this enzyme class is 3,4-dihydroxy-trans-cinnamate:oxygen 3,4-oxidoreductase (decyclizing). This enzyme participates in phenylpropanoid biodegradation.