Objective: To assess the antidiabetic effect of Opuntia dillenii seed oil on rats with diabetes mellitus. Methods: A rat diabetes model was established by intraperitoneal injection of rats with 50 mg/kg streptozotocin. Thirty albino Wistar rats were divided into five groups: the diabetic control group and normal control group were treated only with distilled water, two diabetic groups received 1 and 2 mL/kg of oil per day, respectively, for 30 days and one diabetic group received 2 mg/kg of glibenclamide. In addition, blood glucose was determined weekly. Body weight, average daily food, water intake and urinary volume of each animal were determined before and after the treatment period. After the treatment period, hepatic glycogen was determined using the anthrone reagent, and glycosuria, total cholesterol, triglycerides, alanine aminotransferase, aspartate aminotransferase, urea, creatinine and uric acid were estimated using common clinical diagnostic kits. Results: Oral intake of the oil at 1 and 2 mL/kg for the diabetic animals significantly diminished blood glucose, glycosuria, total cholesterol, triglycerides, alanine aminotransferase, aspartate aminotransferase, urea, creatinine and uric acid, accompanied by a noticeable elevation in the amount of hepatic glycogen in comparison with the diabetic control group. Similarly, Opuntia dillenii seed oil significantly increased the food intake and decreased the urinary volume per day in treated rats of the same groups in comparison with the period before the treatment intervention and attenuated body weight loss in the diabetic rats. Moreover, this effect of the oil was dose dependent. On the other hand, the oil did not affect their need for water. Conclusions: The results show that Opuntia dillenii seed oil has a very important antidiabetic effect on streptozotocin-induced diabetic rats. Hence, we suggest it as a preventive control of diabetes mellitus.