We evaluated the inhibitory effects of plant-based extracts (grape seed, green tea, and white tea) on α-amylase and α-glucosidase activity, glucosidases required for starch digestion. The abundant flavan-3-ol monomers (catechins) in these extracts were also tested for their inhibitory potential and evaluated against the pharmacological glucosidase inhibitor, acarbose. To evaluate relative potency of these extracts and catechins, the concentrations required for 50 and 90% inhibition of enzyme activity were determined. Maximum enzyme inhibition was used to assess an inhibitor’s relative efficacy. Results showed that grape seed extract strongly inhibited both α-amylase and α-glucosidase activity, with equal and much higher potency, respectively, than acarbose. While tea extracts and individual catechin 3-gallates were less effective inhibitors of α-amylase, they were potent inhibitors of α-glucosidase. Our data show that plant extracts containing catechin 3-gallates are potent inhibitors of α-glucosidase, and suggest that procyanidins found in grape seed extract strongly inhibit α-amylase activity.