In this study, the hydrolysis of a flaxseed protein isolate with Alcalase was performed as a strategy to generate antioxidant peptides. A chromatographic separation of the hydrolysate was conducted by RP-HPLC. Both hydrolysate and six collected fractions were subjected to ORAC and FRAP assays to evaluate their antioxidant capacity. The higher antioxidant values were shown by fractions containing predominantly low molecular weight peptides, as it was demonstrated by MALDI analysis. Four peptides were identified by LC-MS/MS and one by Edman degradation. The peptide with sequence GFPGRLDHWCASE was synthesised showing a notable ORAC activity, 3.20 μmol Trolox equivalents/μmol of peptide. This value was higher than that reported for butylated hydroxyanisole. Therefore, the contribution of this peptide to the activity of the fraction where it had been found was 61%. The identified sequences represent an advance in the molecular characterization of the flaxseed protein fraction.This work has received financial support from projects AGL2015-66886-R and FAPESP (2010/52680-7). F.G.D. Silva acknowledges the CNPq for the scholarship granted, and B. Hernández-Ledesma thanks the Ministry of Economy and Competitiveness (MINECO) for her “Ramón y Cajal” post-doctoral contract.Peer Reviewe