In enzymology, an allantoinase (EC 3.5.2.5) is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are (S)-allantoin and H2O, whereas its product is allantoate.

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 (S)-allantoin amidohydrolase.

This enzyme participates in purine metabolism.

This EC 3.5 enzyme-related article is a stub.