Polydopamine (PDA) nanoparticles are versatile structures that can be stabilized with proteiins.In this study, we have demonstrated the feasibility of developping PDA/polypeptides complexe in the shape of nanoparticles. The polypeptide can also renders the nanoparticle functional. Herein, we have developed antimicrobial nanoparticles with a narrow size distribution by decorating the polydopamine particles with a chain-length controlled antimicrobial agent Polyarginine (PAR). The obtained particles were 3.9 ± 1.7 nm in diameter and were not cytotoxic at 1:20 dilution and above. PAR-decorated nanoparticles have demonstrated a strong antimicrobial activity against S. aureus, one of the most common pathogen involved in implant infections. The minimum inhibitory concentration is 5 times less than the cytotoxicity levels. PAR-decorated nanoparticles are incorporated into gelatin hydrogels used as a model of tissue engineering scaffolds and to confer them strong antimicrobial properties without affecting their stability and biocompatibility while improving their mechanical properties (increased storage modulus). Decorated Polydopamine particles can be a versatile tool for functionalization of hydrogels in regenerative medicine applications by providing bioactive properties.