In enzymology, a hydrogen dehydrogenase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are H<sub>2</sub> and NAD<sup>+</sup>, whereas its two products are H<sup>+</sup> and NADH.
This enzyme belongs to the family of oxidoreductases, specifically those acting on hydrogen as donor with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is hydrogen:NAD+ oxidoreductase. Other names in common use include H2:NAD+ oxidoreductase, NAD+-linked hydrogenase, bidirectional hydrogenase, and hydrogenase. This enzyme participates in glyoxylate and dicarboxylate metabolism and methane metabolism. It has 6 cofactors: FAD, Iron, FMN, Flavin, Nickel, and Iron-sulfur.