A new quercetin 3-O-trisaccharide was isolated from the leaves of Rubus crataegifolius (Rosaceae). The structure of this compound was determined as quercetin $3-O-{alpha}-L-arabinofuranosyl-(1{ ightarrow}2)-{eta}-D-xylopyranosyl-(1{ ightarrow}2)-{eta}-D-galactopyranoside$ on the basis of 2D-NMR spectroscopic data. This flavonol glycoside was isolated for the first time from a natural source.