In enzymology, a phloroisovalerophenone synthase (EC 2.3.1.156) is an enzyme that catalyzes the chemical reaction

isovaleryl-CoA + 3 malonyl-CoA ${\displaystyle \rightleftharpoons }$ 4 CoASH + 3 CO₂ + 3-methyl-1-(2,4,6-trihydroxyphenyl)butan-1-one

Thus, the two substrates of this enzyme are isovaleryl-CoA and malonyl-CoA, whereas its 3 products are CoASH, CO₂, and 3-methyl-1-(2,4,6-trihydroxyphenyl)butan-1-one.

This enzyme belongs to the family of transferases, specifically those acyltransferases transferring groups other than aminoacyl groups. The systematic name of this enzyme class is isovaleryl-CoA:malonyl-CoA acyltransferase. Other names in common use include valerophenone synthase, and 3-methyl-1-(trihydroxyphenyl)butan-1-one synthase.