In enzymology, an arginine deiminase (EC 3.5.3.6) is an enzyme that catalyzes the chemical reaction

L-arginine + H₂O ${\displaystyle \rightleftharpoons }$ L-citrulline + NH₃

Thus, the two substrates of this enzyme are L-arginine and H₂O, whereas its two products are L-citrulline and NH₃.

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 L-arginine iminohydrolase. Other names in common use include arginine dihydrolase, citrulline iminase, and L-arginine deiminase. This enzyme participates in arginine and proline metabolism. This enzyme is widely expressed in bacteria, including streptococcus and actinomyces. The bacterial arginine deiminase expression could be regulated by various environmental factors.

Recently, a new enzyme that catalyzes the chemical reaction

L-arginine + 2H₂O ${\displaystyle \rightleftharpoons }$ L-ornithine + 2NH₃ + CO₂ was identified in cyanobacteria^[1] which should be named as arginine dihydrolase.

As of late 2007, 7 structures have been solved for this class of enzymes, with PDB accession codes 1LXY, 1RXX, 1S9R, 2A9G, 2AAF, 2ABR, and 2ACI.