In enzymology, a thiamine oxidase () is an enzyme that catalyzes the chemical reaction
The 3 substrates of this enzyme are thiamine, O<sub>2</sub>, and H<sub>2</sub>O, whereas its two products are thiamine acetic acid and H<sub>2</sub>O<sub>2</sub>.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with oxygen as acceptor. The systematic name of this enzyme class is thiamine:oxygen 5-oxidoreductase. Other names in common use include thiamin dehydrogenase, thiamine dehydrogenase, and thiamin:oxygen 5-oxidoreductase. This enzyme participates in thiamine metabolism. It employs one cofactor, FAD.