Anti-diabetic effect was observed with Thespesia lampas Dalz & Gibs (Family: Malvaceae) when given as a root extract in normal as well as alloxan induced diabetic rats. The effects, however, were more pronounced in diabetic animals in which administration of plant extract for 15 days after alloxan induced diabetes, significantly reduced blood glucose levels. After alloxan induced diabetes it was observed that both standard drug (glibenclamide) and aqueous extract of Thespesia lampas were significantly superior to control in reducing blood sugar on long term treatment (15 days). The aqueous extract of T. lampas (300 and 600 mg/kg) reduced the blood glucose levels from $349.2{pm}7.2$ to $120.7{pm}4.6$ and $346.3{pm}3.4$ to $101.8{pm}6.3$, respectively. The data suggested that T. lampas could be of beneficial in diabetes mellitus in controlling blood sugar. The present investigation established pharmacological evidence to support the folklore claim as an anti-diabetic.