Abstract Pterocarpus santalinoides is used in Nigerian ethnomedicine to treat diabetes mellitus. This study aimed to establish the antidiabetic property of the plant, and isolate and characterize its active principle. Dried and pulverized leaves (500 g) of P. santalinoides were extracted with 1.8 L of 80 % hydromethanol by cold maceration. The dried extract (10 g) was partitioned into n-hexane, ethyl acetate (EtOAc), n-butanol, and water. Antidiabetic activitiy-guided isolation by column chromatographic separation of the EtOAc soluble and purification of the sub-fractions by repeated preparative thin layer chromatography (pTLC) yielded a C-glycosyl flavonoid, identified as isovitexin. The chemical structure was elucidated based on high-resolution mass spectroscopy, 1D, and 2D nuclear magnetic resonance spectroscopic analyses. Alloxan-induced diabetic rat model was adopted for antidiabetic screening. The extract of P. santalinoides (100-200 mg/kg), fraction F4 (50 mg/kg), sub-fraction F4.3 (10 mg/kg), and the semi-purified compound F4.3.2 (5 mg/kg) significantly (p<0.05) reduced the fasting blood glucose of alloxan-induced diabetic rats, causing 48.4, 69.4, 57.7 and 64.5 % antidiabetic activity respectively, compared with > 68 % recorded in glibenclamide (2 mg/kg) control. These results reveal that isovitexin is the antidiabetic principle in P. santalinoides