Sepiapterin reductase (L-threo-7,8-dihydrobiopterin forming) () is an enzyme with systematic name L-threo-7,8-dihydrobiopterin:NADP<sup>+</sup> oxidoreductase. This enzyme catalyses the following two chemical reactions:
This bacterial (Chlorobium tepidum) enzyme catalyses the final step in the de novo synthesis of tetrahydrobiopterin from GTP.