In enzymology, a mycothiol-dependent formaldehyde dehydrogenase () is an enzyme that catalyzes the chemical reaction:
The two substrates of this enzyme are S-(hydroxymethyl)mycothiol and oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>). Its products are S-formylmycothiol, reduced NADH, and a proton.
The enzyme belongs to the family of oxidoreductases, specifically those acting on the aldehyde or oxo group of donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is formaldehyde:NAD+ oxidoreductase (mycothiol-formylating). This enzyme is also called NAD/factor-dependent formaldehyde dehydrogenase or S-(hydroxymethyl)mycothiol dehydrogenase.