Respiratory complex I, EC 7.1.1.2 (also known as NADH:ubiquinone oxidoreductase, Type I NADH dehydrogenase and mitochondrial complex I) is the first large protein complex of the respiratory chains of many organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and translocates protons across the inner mitochondrial membrane in eukaryotes or the plasma membrane of bacteria.

This enzyme is essential for the normal functioning of cells, and mutations in its subunits lead to a wide range of inherited neuromuscular and metabolic disorders. Defects in this enzyme are responsible for the development of several pathological processes such as ischemia/reperfusion damage (stroke and cardiac infarction), Parkinson's disease and others.