In enzymology, juglone 3-monooxygenase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are 5-hydroxy-1,4-naphthoquinone, AH<sub>2</sub>, and O<sub>2</sub>, whereas its 3 products are 3,5-dihydroxy-1,4-naphthoquinone, A, and H<sub>2</sub>O.
This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O<sub>2</sub> as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O miscellaneous. The systematic name of this enzyme class is 5-hydroxy-1,4-naphthoquinone,hydrogen-donor:oxygen oxidoreductase (3-hydroxylating). Other names in common use include juglone hydroxylase, naphthoquinone hydroxylase, and naphthoquinone-hydroxylase.