In enzymology, an isoorientin 3'-O-methyltransferase (EC 2.1.1.78) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + isoorientin ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + isoscoparin

Thus, the two substrates of this enzyme are S-adenosyl methionine and isoorientin, whereas its two products are S-adenosylhomocysteine and isoscoparin.

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:isoorientin 3'-O-methyltransferase. This enzyme is also called isoorientin 3'-methyltransferase.