Diabetes mellitus is a metabolic disorder that impairs glucose homeostasis and can lead to severe complications if poorly managed. Some of these complications may manifest as retinopathy, angiopathy, nephropathy and neuropathy. The methanolic leaf extract of Detarium microcarpum was investigated for possible blood glucose lowering activity in alloxan-induced diabetic rats. Diabetes was induced by intra- peritoneal injection of 150 mg/kg of alloxan monohydrate. Body weight and blood glucose levels of control and diabetic rats were monitored. The effect of the extract on internal organs (heart, lung, liver and kidney) of the rat was also determined. The acute toxicity and qualitative phytochemical analysis for the identification of various plants constituents were also carried out using standard methods. The preliminary phytochemical screening of the extract revealed the presence of terpenoids, flavonoids, saponins, tannins and steroids. The methanolic leaf extract of Detarium microcarpum at 500, 750, 1000 mg/kg body weight exhibited a dose dependent significant reduction of blood glucose level. A significant increase (p<0.05) in the body weight of diabetic rats treated with Detarium microcarpum extract was also observed when compared with the standard drug (Glibenclimide). The result of the acute toxicity studies revealed that none of the organs showed any histopathological presentations within the period of observation. The methanolic leaf extract of Detarium microcarpum showed good antidiabetic activity with minimal toxicity and is therefore its ethnomedicinal use in the management of diabetes may be justified.