In enzymology, a flavonol-3-O-glucoside L-rhamnosyltransferase (EC 2.4.1.159) is an enzyme that catalyzes the chemical reaction

UDP-L-rhamnose + a flavonol 3-O-D-glucoside ${\displaystyle \rightleftharpoons }$ UDP + a flavonol 3-O-[beta-L-rhamnosyl-(1->6)-beta-D-glucoside]

Thus, the two substrates of this enzyme are UDP-L-rhamnose and flavonol 3-O-D-glucoside, whereas its two products are UDP and [[flavonol 3-O-[beta-L-rhamnosyl-(1->6)-beta-D-glucoside]]].

This enzyme belongs to the family of glycosyltransferases, specifically the hexosyltransferases. The systematic name of this enzyme class is UDP-L-rhamnose:flavonol-3-O-D-glucoside 6''-O-L-rhamnosyltransferase. Other names in common use include uridine diphosphorhamnose-flavonol 3-O-glucoside, rhamnosyltransferase, and UDP-rhamnose:flavonol 3-O-glucoside rhamnosyltransferase. This enzyme participates in flavonoid biosynthesis.

• Wiermann R (1982). "Formation of flavonol 3-O-diglycosides and flavonol 3-O-triglycosides by enzyme extracts from anthers of Tulipa cv apeldoorn - characterization and activity of 3 different O-glycosyltransferases during anther development". Z. Naturforsch. C. 37: 587–599.
• Jones P, Messner B, Nakajima J, Schaffner AR, Saito K (2003). "UGT73C6 and UGT78D1, glycosyltransferases involved in flavonol glycoside biosynthesis in Arabidopsis thaliana". J. Biol. Chem. 278 (45): 43910–8. doi:10.1074/jbc.M303523200. PMID 12900416.