Display omitted •Poly(methacrylic acid) star-polyelectrolytes are synthesized via RAFT polymerization.•Their electrostatic complexation with lysozyme is investigated.•The properties of the formed complexes depend on the ratio of the two components.•The molar mass of the star-polymer also affects the complexes’ structure.•The protein does not denature upon complexation. Protein-polyelectrolyte complexes are of great interest due to their applications in both biological and industrial systems. This study focuses on investigating the electrostatic complexation between (hen egg white) lysozyme (LYZ), a well-studied enzyme, and two star-shaped poly(methacrylic acid) (PMAA) polymers with different arm molar masses, namely S-PMAA5K and S-PMAA10K. Depending on the mixing ratio of the two components (protein/star polyelectrolyte) either stable solutions were formed or macroscopic phase separation, i.e. coacervation, was observed. Dynamic and electrophoretic light scattering results reveal that the overall physicochemical properties of the complexes and the strength of the interaction depends on the ratio of the two components as well as on the molar mass of the polymer, with the S-PMAA10K series exhibiting stronger interactions compared to S-PMAA5K. Additional information regarding the molar mass of the star polyelectrolytes and the corresponding complexes with lysozyme were acquired by analytical ultracentrifugation. The morphology of the complexes was further elucidated through atomic force and cryo-transmission electron microscopy showing globular raspberry-like objects. Moreover, fluorescence and infrared spectroscopic techniques were employed to explore potential conformational changes of the protein upon complexation, with both techniques indicating that the protein does not undergo significant structural changes. Furthermore, UV–Vis spectroscopy was utilized to quantify the amount of protein present in the supernatant of the coacervated complex solutions, thus allowing the quantification of the unbound protein remaining in the solution after complex formation.