To mimic the quinone hardening of extracellular proteins in invertebrates, we investigated an enzyme-free crosslinking of gelatin by HQ in a neutral aqueous phase. The mixture was rapidly transformed to a yellowish brown, thermally and mechanically stable hydrogel in the presence of a simple copper(II) salt. A dehydrated thin film made of the mixture was flexible, tough, and showed a large ultimate breaking force. Physicochemical examination of the gel suggested that the basic amino acid residues (lysine, hydroxylysine, and histidine) of the protein were modified by the quinone ring to form 2-6 crosslinks per protein. The enzyme-free crosslinking reaction is discussed with consideration of a copper(II) ion-catalyzed oxidation of HQ and the hydroquinone/protein adducts.