In enzymology, a cob(II)alamin reductase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are cob(I)alamin and NAD<sup>+</sup>, whereas its 3 products are cob(II)alamin, NADH, and H<sup>+</sup>.
This enzyme belongs to the family of oxidoreductases, specifically those oxidizing metal ion with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is cob(I)alamin:NAD+ oxidoreductase. Other names in common use include vitamin B12r reductase, B12r reductase, and NADH2:cob(II)alamin oxidoreductase. This enzyme participates in porphyrin and chlorophyll metabolism. It employs one cofactor, FAD.