The bark of Ceriops tagal has been used in the Philippines for folk treatment of diabetes mellitus (DM). Through an α-glucosidase inhibition assay guided isolation, constituents from the detannified methanolic bark extract were isolated and identified to be methyl 2,4-dihydroxy-3,6- dimethyl benzoate (1), isopimar-7-ene-15S,16-diol (2), betulinic acid (3), lupeol (4), and methyl 2,4-dihydroxy-6-methyl benzoate (5). Compounds 3 (IC50 = 5.31 μM) and 4 (IC50 = 55.84 μM) showed a more potent activity against α-glucosidase than the positive control, 1-deoxynojirimycin(IC50 = 79.16 μM). This is the first report on the presence of the weakly active compounds (1,2, and 5) (IC50 > 75 μM) in the genus Ceriops and the first report on the biological activity ofCompound 2. Enzyme kinetic studies suggested that betulinic acid is an uncompetitive type ofinhibitor, with lupeol as a non-competitive type. This study provides evidence on the possiblemechanism of the antidiabetic property of C. tagal associated with its traditional use. It supportsthe possible utilization of the bark for development to prevent or treat diabetes.