In enzymology, (S)-usnate reductase () is an enzyme that catalyzes the chemical reaction
The enzyme converts the precursor (6R)-2-acetyl-6-(3-acetyl-2,4,6-trihydroxy-5-methylphenyl)-3-hydroxy-6-methylcyclohexa-2,4-dien-1-one into (S)-usnic acid using oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>) as cofactor and forming an ether bond.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD or NADP as acceptor. The systematic name of this enzyme class is reduced-(S)-usnate:NAD oxidoreductase (ether-bond-forming). This enzyme is also called L-usnic acid dehydrogenase.