In enzymology, a methanethiol oxidase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are methanethiol, O<sub>2</sub>, and H<sub>2</sub>O, whereas its 3 products are formaldehyde, hydrogen sulfide, and H<sub>2</sub>O<sub>2</sub>.
This enzyme belongs to the family of oxidoreductases, specifically those acting on a sulfur group of donors with oxygen as acceptor. The systematic name of this enzyme class is methanethiol:oxygen oxidoreductase. Other names in common use include methylmercaptan oxidase, methyl mercaptan oxidase, (MM)-oxidase, and MT-oxidase.