In enzymology, a tropinone reductase I () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are tropine and NADP<sup>+</sup>, whereas its 3 products are tropinone, NADPH, and H<sup>+</sup>.
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 tropine:NADP<sup>+</sup> 3alpha-oxidoreductase. Other names in common use include tropine dehydrogenase, tropinone reductase (ambiguous), and TR-I. This enzyme participates in alkaloid biosynthesis ii.