Objective To investigate the antidiabetic activity of Ocimum tenuiflorum L. (O. tenuiflorum) leaves used in the traditional medicine management of diabetes in Malaysia. Methods O. tenuiflorum leaves were extracted sequentially with hexane, chloroform, ethyl acetate, methanol, and water. The extracts were evaluated in terms of antidiabetic activity by using acute, subcutaneous glucose tolerance, and sub-chronic tests in streptozotocin-induced diabetic rats. The extracts were also subjected to phytochemical analyses. Results With an acute dose (1 g/kg), the methanol extracts showed significant reduction (31%) in fasting blood glucose (FBG) of the streptozotocin-induced diabetic rats. The FBG-decreasing effect of ethyl acetate extract was more rapid than that of the other extracts; the decreasing rates were 20% after 2 h, 21% after 3 h, and 8% after 5 and 7 h. After 7 h (31%), the effect of methanol extract on FBG was significantly lower than that of metformin. In the subcutaneous glucose tolerance test, only methanol and hexane extracts showed the similarity of metformin in diabetic rats. After 14 days, the effects of these extracts were similar to those of metformin (63.33%). The total flavonoid and phenolic contents of extracts decreased as the polarity of the extraction solvent increased. Conclusions The results obtained provide support for a possible use of O. tenuiflorum leaves in managing hyperglycemia and preventing the complications associated with it in type 2 diabetic.