NADH dehydrogenase (, cytochrome c reductase, type 1 dehydrogenase, beta-NADH dehydrogenase dinucleotide, diaphorase, dihydrocodehydrogenase I dehydrogenase, dihydronicotinamide adenine dinucleotide dehydrogenase, diphosphopyridine diaphorase, DPNH diaphorase, NADH diaphorase, NADH hydrogenase, NADH oxidoreductase, NADH-menadione oxidoreductase, reduced diphosphopyridine nucleotide diaphorase) is an enzyme with systematic name NADH:acceptor oxidoreductase. This enzyme catalyses the following chemical reaction

Complex I (also referred to as NADH:ubiquinone oxidoreductase or, especially in the context of the human protein, NADH dehydrogenase (ubiquinone)) is an enzyme of the respiratory chains of myriad organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and, in eukaryotes, it is located in the inner mitochondrial membrane. It is one of the "entry enzymes" of cellular respiration or oxidative phosphorylation in the mitochondria.