OBJECTIVE: To characterize a glycosyltransferase (RpUGT1) from Rheum palmatum and investigate its specificity toward flavonoid compounds. METHODS: The RpUGT1 was expressed in Escherichia coli and screened for catalytic activity against a range of flavonoid substrates using a high-throughput HPLC assay method. Mass spectrometry (MS) and nuclear magnetic resonance (NMR) were used to determine the structure of the product. Homology modeling, molecular docking analyses and site-directed mutagenesis studies were conducted to identify key residues responsible for its function. RESULTS: The recombinant RpUGT1 protein exhibited catalytic activity towards various flavonoids. Notably, RpUGT1 catalyzed the glycosylation of isorhamnetin to form 3-O-glucoside and kaempferol to form 7-O-glucoside, utilizing uridine diphosphate (UDP) glucose as the sugar donor. The homology modeling and molecular docking analyses identified key residues responsible for its activity. Subsequent site-directed mutagenesis studies highlighted the crucial role of K307 in catalysis. CONCLUSION These discoveries offer valuable perspectives on the role of the UGT family and establish a groundwork for forthcoming research on the synthesis of flavonoids in plants.