Hypoglycemic effect of onion skin extract in vitro and in vivo was investigated. A methanol extract of onion skin inhibited yeast $alpha$-glucosidase with an $IC_{50}$ of 0.159 mg/mL. A single oral administration of the onion skin extract (500 mg/kg) significantly lowered the postprandial area under the glucose response curve to starch (1 g/kg, p<0.05). Three-week-old db/db mice were fed an AIN-93G diet or a diet supplemented with a 0.5% onion skin extract for 7 weeks after a 1-week adaptation period. Consumption of onion skin extract significantly reduced the levels of plasma glucose, insulin, and blood glycated hemoglobin as compared with the control group (p<0.05). These findings suggest that onion skin is effective in controlling hyperglycemia in animal models of type 2 diabetes mellitus, at least in part by inhibiting $alpha$-glucosidase activity.