In enzymology, a hydroxyisourate hydrolase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are 5-hydroxyisourate and H<sub>2</sub>O, whereas its product is 5-hydroxy-2-oxo-4-ureido-2,5-dihydro-1H-imidazole-5-carboxylate.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in cyclic amides. The systematic name of this enzyme class is 5-hydroxyisourate amidohydrolase. Other names in common use include HIUHase, and 5-hydroxyisourate hydrolase. This enzyme participates in purine metabolism.
As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .