Harnessing melanins to scavenge free radicals in vivo may yield treatment methods for inflammatory disorders. Furthermore, elucidation of the mechanism underlying melanin-mediated suppression of free radicals, which is yet unclear, is warranted. Herein, we chemically synthesized melanin-mimetic nanoparticles (MeNPs) and investigated the mechanism underlying their use. MeNPs efficiently suppressed hydroxyl radicals by converting some MeNP hydroxyl groups to ketone groups. Furthermore, they suppressed hydroxyl radicals produced by lipopolysaccharide-treated Kupffer cells involved in hepatic cirrhosis pathogenesis, without causing significant cytotoxicity. The present results indicate the suitability of MeNPs to treat hepatic cirrhosis; however, further in vivo studies are warranted to determine their treatment efficacy.