Abstract The pericarp of Trapa natans L., an annual aquatic floating herb belonging to Lythraceae family, is used as a folk medicine in China. In this study, extracts of Trapa natans pericarp were tested both in vitro and in vivo through a high-fat diet with a single medium dosage streptozotocin injection induced type 2 diabetic mice. Different solvent extracts of Trapa natans pericarp showed α-amylase and α-glucosidase inhibitory activity. After four weeks administration, the ethyl acetate extract of Trapa natans pericarp (50 and 100 mg/kg b.w.) reduced fasting blood glucose level, ameliorated oral glucose tolerance and insulin resistance, improved serum lipids alterations in type 2 diabetic mice as well. Additionally, ethyl acetate extract significantly elevated the insulin receptor substrate 1 and Akt serine/threonine kinase phosphorylation compared to diabetic group. HPLC-MS and HPLC-DAD analysis showed that the ethyl acetate extract was rich in hydrolysable tannins. Results support the notion that Trapa natans pericarp extract has a potential hypoglycemic activity.