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The present study aimed to investigate the effect of covalent coupling with the dietary polyphenol rosmarinic acid (RA) on the structure and properties of β-lactoglobulin (β-LG). β-LG-RA complexes were prepared using the free radical and alkaline methods. The structural changes of β-LG after coupling with RA were analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier transform infrared (FTIR) spectroscopy, ultraviolet (UV) absorption spectroscopy, fluorescence spectroscopy, and circular dichroism (CD) spectroscopy. The antioxidant activity of β-LG and its complexes with RA was assessed using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) radical cation scavenging methods. Furthermore, their binding capacity to serum-specific immunoglobulin G (IgG) were evaluated using enzyme-linked immunosorbent assay (ELISA). The results revealed that the secondary structure of β-LG transformed from α-helix to β-sheet and random coil after RA covalent conjugation. The protein underwent changes in its secondary and tertiary structures. Additionally, compared to β-LG, the β-LG-RA covalent conjugates showed significantly enhanced antioxidant activity and reduced binding ability with serum specific IgG. In conclusion, covalent coupling with RA led to a significant improvement in the antioxidant activity of β-LG and a significant decrease in its binding capacity to serum-specific IgG by introducing phenolic hydroxyl groups.