The prevalence of diabetes mellitus is increasing worldwide. The objective is to investigate and characterize the alpha-amylase inhibition principles of the leaves of Ficus sur Forssk (Moraceae) used in ethnomedicine for management of diabetes mellitus. Powdered leaves were successively macerated with N-hexane, chloroform and 70% ethanol respectively for three consecutive days. The extracts were assessed for phytochemicals and inhibition of alpha-amylase. Alpha-amylase inhibition was assessed using porcine α-amylase. Bioactive n-hexane extract was fractionated on column chromatography packed with Silica Gel G (mesh 60-120) and eluted with gradient mixture of hexane, ethyl acetate and ethanol. Active fractions (F1 - F5) were purified on preparative thin layer chromatography. Active pure compounds were assessed for bioactivity and identified by spectroscopy (UV, IR, MS and NMR). The n-hexane extracts of the plant exhibited highest significant (p < 0.05) inhibition. Fraction F4 was the most active and was and compound FB was characterized from it as a novel abietane-type diterpene (4,7,10-b-trimethyl-benzofuro[c]-6,6-a,8,9,10,10-a-hexahydroisochromene-7-carboxylic acid) with 55% inhibition of alpha-amylase at 50µg/ml. This work reports for the first time a novel diterpenoid from Ficus sur with alpha-amylase inhibition activity.