In enzymology, tryptophan dehydrogenase () is an enzyme that catalyzes the chemical reaction
The three substrates of this enzyme are L-tryptophan, oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>), and water. Its products are indole-3-pyruvic acid, ammonia, reduced NADH, and a proton. Nicotinamide adenine dinucleotide phosphate can be used as an alternative cofactor.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-NH<sub>2</sub> group of donors with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor. The systematic name of this enzyme class is L-tryptophan:NAD(P)+ oxidoreductase (deaminating). Other names in common use include NAD(P)<sup>+</sup>-L-tryptophan dehydrogenase, L-tryptophan dehydrogenase, L-Trp-dehydrogenase, and TDH. This enzyme has at least one effector, calcium.