Withania frutescens L. is a wild perennial woody plant used by the local population for diverse therapeutic purposes. This work aims to study for the first time the potential inhibitory effect of this plant hydroethanolic extract on α-amylase and α-glucosidase activities using in vitro methods and its antidiabetic and antihyperglycemic activities using alloxan-induced diabetic mice as a model for experimental diabetes. Two doses were selected for the in vivo study (200 and 400 mg/kg) and glibenclamide, a well-known antidiabetic drug (positive control) in a subacute study (28 days) where the antihyperglycemic activity was also assessed over a period of 12 h on diabetic mice. The continuous treatment of diabetic mice with the extract of Withania frutescens for 4 weeks succeeded to slowly manage their high fasting blood glucose levels (after two weeks), while the antihyperglycemic test result revealed that the extract of this plant did not control hyperglycemia in the short term. No toxicity signs or death were noted for the groups treated with the plant extract, and it shows a protective effect on the liver and kidney. The in vitro assays demonstrated that the inhibition of alpha-amylase and alpha-glucosidase might be one of the mechanisms of action exhibited by the extract of this plant to control and prevent postprandial hyperglycemia. This work indicates that W. frutescens have an important long term antidiabetic effect that can be well established to treat diabetes.