The in vitro inhibitory activities of different seed extracts prepared from cranberry bean mutant SA-05 and its wild-type variety Hwachia against aldose reductase, α-glucosidase and α-amylase were examined. The results indicated that the polyphenolics-rich extracts obtained using 800 g kg−1 methanol and 500 g kg−1 ethanol demonstrated inhibitory activities against aldose reductase (IC50 of 0.36–0.46 mg mL−1) and α-glucosidase (IC50 of 1.32–1.94 mg mL−1). The 500 g kg−1 ethanol extracts also showed α-amylase inhibitory activities (IC50 of 70.11–80.22 μg mL−1). Subsequent extracts, prepared further with NaCl and H2O from precipitates of 800 g kg−1 methanol or 500 g kg−1 ethanol extracts, exhibited potent α-amylase inhibitory activities (IC50 of 17.68–38.68 μg mL−1). A combination of 500 g kg−1 ethanol extraction plus a subsequent H2O extraction produced highest polyphenolics and α-amylase inhibitors. The SA-05 α-amylase inhibitor extracts showed greater inhibitory activities than that of Hwachia. Thus, cranberry bean mutant SA-05 is an advantageous choice for producing anti-hyperglycaemic compounds.