4-hydroxymandelate synthase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are 4-hydroxyphenylpyruvic acid and oxygen. Its products are (S)-4-hydroxymandelic acid and carbon dioxide.
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 4-hydroxyphenylpyruvate:oxygen oxidoreductase (decarboxylating). This enzyme is also called 4-hydroxyphenylpyruvate dioxygenase II.