Phytochemical investigations on the most active EtOAc extract led to the isolation of 9 flavonoids [quercetin (1), quercetin 3-O-beta-glucopyranoside (2), quercetin 3-O-beta-galactopyranoside (3), quercetin 3-O-(2 ''-O-galloyl)-beta-glucopyranoside (4), quercetin 3-O-(2 ''-O-galloyl)-beta-galactopyranoside (5), quercetin 3-O-(6 ''-O-galloyl)-beta-glucopyranoside (6), quercetin 3-O-(6 ''-O-galloyl)-beta-galactopyranoside (7), quercetin-3-O-alpha-arabinofuranoside (8), and quercetin-3-O-alpha-rhamnopyranosyl-(1 -> 6)-beta-glucopyranoside (9)] and a simple phenolic compound (methyl gallate) (10). The structures of the compounds were elucidated by 1- and 2-dimensional NMR techniques (H-1, C-13, COSY, HMBC, and HMQC) and ESI-TOF-MS spectrometry. H2O2-induced lipid peroxidation inhibitor effects in human red blood cells of different extracts of G. tuberosum L. subsp. tuberosum aerial parts were also investigated. All the compounds tested showed antioxidant activity.