In enzymology, a galactitol-1-phosphate 5-dehydrogenase (EC 1.1.1.251) is an enzyme that catalyzes the chemical reaction

galactitol-1-phosphate + NAD⁺ ${\displaystyle \rightleftharpoons }$ L-tagatose 6-phosphate + NADH + H⁺

Thus, the two substrates of this enzyme are galactitol-1-phosphate and NAD⁺, whereas its 3 products are L-tagatose 6-phosphate, NADH, 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 galactitol-1-phosphate:NAD⁺ oxidoreductase. This enzyme participates in galactose metabolism. It employs one cofactor, zinc.