Polyphenol-rich extracts from a range of berries inhibited α-amylase in vitro, but the most effective were from raspberry and rowanberry (IC50 values of 21.0 and 4.5 μg/mL, respectively). The inhibitory components were examined by different approaches. Extracts from yellow and red raspberries were equally able to inhibit α-amylase. Because the yellow raspberry extracts effectively lacked anthocyanins, this suggested that they were not crucial for amylase inhibition. Notably, however, higher levels of other phenolic components in yellow raspberries (particularly, ellagitannins) did not increase amylase inhibition. Amylase inhibition in rowanberry was recovered in a fraction enriched in proanthocyanidins (PACs). Inhibition was ameliorated by bovine serum albumin, suggesting that PACs acted by binding to amylase. Co-incubation of rowanberry PACs with acarbose reduced the concentration of acarbose required for effective amylase inhibition. Such synergistic interactions could have implications for the current clinical use of acarbose for postprandial glycaemic control in type-2 diabetics.