There is a growing interest in utilizing renewable resources and exploiting biological reactions for environmentally friendly products and processes. We report the use of the enzyme tyrosinase to graft the natural phenol, catechin, onto the biopolymer chitosan. Chemical evidence for grafting was obtained by UV/visible spectrophotometry and electrospray mass spectrometry. Rheological measurements demonstrate that the catechin-modified chitosan behaves as an associative thickener. Specifically, the viscosity increases dramatically with concentration of this modified chitosan. Furthermore, when the catechin-modified chitosan is dissolved at low concentrations (0.6% w/w), steady shear measurements show shear thinning behavior, while oscillatory measurements show weak gel behavior. These results demonstrate the potential for utilizing renewable resources and biochemical processing to functionalize biopolymersto offertechnically useful properties. To suggest the relative environmental impacts of chitosan derivatives with existing water-soluble polymers, we used the framework of a life cycle assessment.