The maple (Acer) genus is a reported source of bioactive (poly)phenols, including gallotannins, but several of its members, such as the sycamore maple (A. pseudoplatanus), remain uninvestigated. Herein, thirty-nine compounds, including a new gallotannin, 1,2,3-tri-O-galloyl-6-O-(p-hydroxybenzoyl)-beta-D-glucopyranoside (1), and thirty-eight (2-39) known compounds, consisting of four gallotannins, one ellagitannin, thirteen flavonoids, eight hydroxycinnamic acids, ten benzoic acid derivatives, and two sesquiterpenoids, were isolated from sycamore maple leaves. Their structures were determined based on NMR and mass spectral analyses. The isolates were evaluated for a-glucosidase inhibitory and antioxidant activities. Among the isolates, the gallotannins were the most potent alpha-glucosidase inhibitors with thirteen-fold more potent activity compared with the clinical drug, acarbose (IC50 = 16-31 vs. 218 mu M). Similarly, the gallotannins showed the highest antioxidant activities, followed by the other phenolic sub-classes, while the sesquiteroenoids were inactive.