An environmentally benign surface modification process for plastic films was demonstrated by fabricating composite coatings through layer-by-layer assembly with green solid materials: aqueous dispersions of two kinds of crystalline polysaccharide nanofibrils. Anionic cellulose nanofibrils were obtained by the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation of native cellulose, while cationic beta-chitin nanofibrils were prepared by the protonation of squid pen chitin. Uniform layer depositions, driven by electrostatic attraction and enhanced by hydrogen bonding, were observed on silicon wafers and then reproduced onto poly(ethylene terephthalate) films. Contact angle measurements, and: dyeing tests on the resulting films revealed their hydrophilic nature and the sorption of both charged and uncharged substances. Antireflection properties were also confirmed via the light transmittance measurements. As might be presumed from all these properties, this composite coating exhibited its unique behavior largely due to its structure, which was distinct from both those of nanofibril cast films and polymer films.