Amaranthus spinosus is a medicinal plant belonging to Amaranthaceae family and used traditionally to treat diabetes mellitus. The aim of this study was to evaluate the antidiabetic activity of this plant in diabetic STZ rats. The antidiabetic activity of the hydroethanolic extract of the roots of A. Spinosus was evaluated in diabetic STZ rats for 21 days. The urinary and biochemical parameters were evaluated at the start and at the end of the treatment. The hematological and histopathological parameters were evaluated just at the end of treatment. The results showed that A. Spinosus extract 500 mg/kg b.w and Glibenclamide used as reference drug caused a significant (p < 0.0001) decrease in oral glucose tolerance and basal blood glucose levels in diabetic STZ rats at the 2^nd and 3^rd week of treatment. At the end of treatment, food and water intake, body weight, urinary, biochemical and haematological parameters were significantly (p < 0.0001) returned to normal in the treated groups. In the liver, kidneys and eyes, a significant decrease (p < 0.0001) in MDA and an increase in GSH was observed in treated rats. Histopathological studies of the kidneys and eyes revealed no damage in rats treated with extract of A. spinosus and glibenclamide in contrast to glomerulosclerosis and macular edema seen in untreated diabetic rats. These results show that A. spinosus roots has bioactive compounds and the extract 500 mg/kg b.w can be administered orally to treat type II diabetes and its complications related to oxidative stress induced by hyperglycemia.