The potent α-glucosidase inhibitor (compound I) was isolated from coffee brews by the activity-based fractionation and identified as a β-carboline alkaloid norharman (9H-pyrido[ 3.4-b]indole) on the basis of mass spectroscopy and nuclear magnetic resonance spectra ((1)H NMR, (13)C NMR, and COSY). The norharman showed a potent inhibition against α-glucosidase enzyme in a concentration dependent manner with an IC50 value of 0.27 mM for maltase and 0.41 mM for sucrase, respectively. A Lineweaver-Burk plot revealed that norharman inhibited α-glucosidase enzyme uncompetitively, with a Ki value of 0.13 mM.