In enzymology, hydroxymethylglutaryl-CoA reductase () is an enzyme that catalyzes the chemical reaction
The substrates of this enzyme are mevalonic acid, coenzyme A, and oxidised nicotinamide adenine dinucleotide (NAD<sup>+</sup>). Its products are 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA), reduced NADH, and two protons.
This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD<sup>+</sup> or NADP<sup>+</sup> as acceptor. The systematic name of this enzyme class is (R)-mevalonate:NAD<sup>+</sup> oxidoreductase (CoA-acylating). Other names in common use include beta-hydroxy-beta-methylglutaryl coenzyme A reductase, beta-hydroxy-beta-methylglutaryl CoA-reductase, 3-hydroxy-3-methylglutaryl coenzyme A reductase, and hydroxymethylglutaryl coenzyme A reductase.