In enzymology, a guanidinobutyrase () is an enzyme that catalyzes the chemical reaction
Thus, the two substrates of this enzyme are 4-guanidinobutanoate and H<sub>2</sub>O, whereas its two products are 4-aminobutanoate and urea.
This enzyme belongs to the family of hydrolases, those acting on carbon-nitrogen bonds other than peptide bonds, specifically in linear amidines. The systematic name of this enzyme class is 4-guanidinobutanoate amidinohydrolase. Other names in common use include gamma-guanidobutyrase, 4-guanidinobutyrate amidinobutyrase, gamma-guanidinobutyrate amidinohydrolase, G-Base, GBH, and guanidinobutyrate ureahydrolase. This enzyme participates in urea cycle and metabolism of amino groups. It employs one cofactor, manganese.