In enzymology, a L-idonate 5-dehydrogenase (EC 1.1.1.264) is an enzyme that catalyzes the chemical reaction

L-idonate + NAD(P)+ ${\displaystyle \rightleftharpoons }$ 5-dehydrogluconate + NAD(P)H + H⁺

The 3 substrates of this enzyme are L-idonate, NAD⁺, and NADP⁺, whereas its 4 products are 5-dehydrogluconate, NADH, NADPH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on the CH-OH group of donor with NAD⁺ or NADP⁺ as acceptor. The systematic name of this enzyme class is L-idonate:NAD(P)⁺ oxidoreductase.