In enzymology, a S-adenosylhomocysteine deaminase (EC 3.5.4.28) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-homocysteine + H₂O ${\displaystyle \rightleftharpoons }$ S-inosyl-L-homocysteine + NH₃

Thus, the two substrates of this enzyme are S-adenosyl-L-homocysteine and H₂O, whereas its two products are S-inosyl-L-homocysteine and NH₃.

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 S-adenosyl-L-homocysteine aminohydrolase. This enzyme is also called adenosylhomocysteine deaminase.

• Zulty JJ, Speedie MK (1989). "Purification and characterization of S-adenosylhomocysteine deaminase from streptonigrin-producing Streptomyces flocculus". J. Bacteriol. 171 (12): 6840–4. PMC 210584. PMID 2592350.