The structure of human serum albumin (HSA) in aqueous solutions in the presence of catechin at a constant molar ratio [HSA]:[Cat]=1:10 and varying the concentration of cobalt ions within [Co2+]:[HSA] from 0 to 100 is studied in this work. The study of the secondary structure of the protein is carried out by FTIR spectroscopy with deconvolution of the Amide I band. Changes in the tertiary structure of the protein are recorded by UV absorption and fluorescence spectra. It was found that at concentration ratios of [HSA]:[Co2+] up to 1:100, there are no disturbances in the globular structure of the protein. There are a decrease in the number of α-helices and a growth in the content of β-layers in the protein structure with an increase in the concentration of cobalt cations. When HSA interacts with catechin, spectral changes are observed, indicating the formation of a complex. Presumably, complex formation leads to quenching of the fluorescence of both compounds. The cause of protein fluorescence quenching can be either a violation of its tertiary structure or the direct binding of catechin and cobalt cations to HSA near aromatic amino acid residues. The value of the zeta potential of protein particles in solution, determined by the negative charge density on HSA, decreases with increasing concentration of CoCl2 in solution, approaching 0 at [Co2+]:[HSA]=100. Catechin does not hinder from the complex formation of HSA with Co2+.