In enzymology, a GTP cyclohydrolase IIa () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are GTP and H<sub>2</sub>O, whereas its two products are 2-amino-5-formylamino-6-(5-phosphoribosylamino)pyrimidin-4(3H)-one and phosphate.
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 GTP 8,9-hydrolase (phosphate-forming).