In enzymology, a sepiapterin deaminase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are sepiapterin and H<sub>2</sub>O whereas its two products are xanthopterin-B2 and NH<sub>3</sub>.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amidines. The systematic name of this enzyme class is sepiapterin aminohydrolase.