AbstractFacile green synthesis of gold nanoparticles (AuNPs) on cellulose fiber was successfully achieved by reducing chloroauric acid (HAuCl4·3H2O) by means of unbleached kraft pulp. A significant color change in pulp fiber indicating the in-situ formation of gold was observed with one-step synthesis in an autoclave. As-prepared AuNP-cellulose fiber nanocomposites were thoroughly characterized by UV–Vis diffuse reflectance spectroscopy (DRS), X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Gold nanoparticles were uniformly dispersed on the surface of the fiber by the bio-reduction of Au3+ from metal salt to Au0 with the α-carbonyl group and conjugated carbonyl of phenolic groups of lignin. The AuNPs formed on cellulose fibers were estimated to have average sizes of approximately 12.5, 12.4, 16.4, and 21.0 nm, depending on the concentration of Au3+ involved in the synthesis.