In enzymology, a glucuronolactone reductase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are L-gulono-1,4-lactone and oxidised nicotinamide adenine dinucleotide phosphate (NADP<sup>+</sup>). Its products are [-glucurono-3,6-lactone, reduced NADPH, and a proton.
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 L-gulono-1,4-lactone:NADP<sup>+</sup> 1-oxidoreductase. Other names in common use include GRase, and gulonolactone dehydrogenase. This enzyme participates in ascorbate and aldarate metabolism.