Bioassay-guided fractionation of the methanol extract of Glochidion hypoleucum (Miq.) Boerl leaves led to the isolation of five polyphenolic compounds, methyl gallate, gallic acid, apigenin-8-C-β-D-glucopyranoside (vitexin), luteolin-8-C-β-D-glucopyranoside (orientin), and luteolin-6-C-β-D-glucopyranoside (isoorientin). The chemical structures of the isolated compounds were determined using spectroscopic (NMR, UV-Vis, IR) and mass spectrometric techniques. The antioxidative properties of the methanol extract and isolated polyphenols were evaluated using 1,1-diphenyl-2-picrylhydrazyl (DPPH) for radical scavenging activity and 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) to measure the level of reactive oxygen species (ROS). With the exception of vitexin, the crude methanol extract and the polyphenolic compounds inhibited DPPH radicals with IC50 values ranging from 2.46 ± 0.05 to 40.0 ± 0.3 μg/mL. In addition, the crude methanol extract attenuated H202-induced intracellular ROS production in a dose-dependent manner in HEK-293 cells. Gallic acid and isoorientin significantly reduced the ROS level in HEK-293 cells at a concentration of 20 μM.