Senna siamea Lam Irwin et Barneby is used as a medicinal plant especially its leaf, fruit, and stem bark. In this work, the leaf of Senna siamea was extracted using 85 % methanol by maceration method, screened, and investigated for phytochemical constituents, acute toxicity was evaluated using Lorke’s method and an antidiabetic study was carried out using alloxan-induced Wistar mice. The result of the phytochemical screening revealed the presence of carbohydrates, tannins, saponins, steroid/terpenes, terpenoids, cardenolides and alkaloids, while phlorotannins and soluble starch were absent. The acute toxicity study revealed no death was recorded on the administration of 10, 100 and 1000 mg/kg dose of the leaf methanol extracts via both the oral and intraperitoneal routes in phase I. But death was recorded in phase two, when an extract dose of 5000 mg/kg of the leaf extracts was administered intraperitoneally. Thus, LD50 of the leaf extracts in rats administered via the oral route was ≥5000 mg/kg while the intraperitoneal route was calculated as 3807 mg/kg. The antidiabetic study revealed that the extract at doses of 200 mg/kg and 400 mg/kg bd.wt produced a significant (p<0.05) reduction in the fasting blood glucose (FBG) of the animals with reductions of 61.01 % and 34.84 % respectively, while both the negative and positive controls had 00.00 % and 76.68 %. The result shows that the effect of the extract was not dose-dependent since the extract of 200 mg/kg exerted a more significant antidiabetic effect on the mice. Thus, the study justified the traditional use of the plant for the management of diabetes.