In enzymology, an apigenin 4'-O-methyltransferase (EC 2.1.1.75) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + 5,7,4'-trihydroxyflavone ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + 4'-methoxy-5,7-dihydroxyflavone

Thus, the two substrates of this enzyme are S-adenosyl methionine and 5,7,4'-trihydroxyflavone (apigenin), whereas its two products are S-adenosylhomocysteine and 4'-methoxy-5,7-dihydroxyflavone (acacetin).

This enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:5,7,4'-trihydroxyflavone 4'-O-methyltransferase. Other names in common use include flavonoid O-methyltransferase, and flavonoid methyltransferase. This enzyme participates in flavonoid biosynthesis.

• Kuroki G, Poulton JE (1981). "The para-O-methylation of apigenin to acacetin by cell-free extracts of Robinia pseudoacacia L". Z. Naturforsch. C: Biosci. 36: 916–920.