Interest in exploring high-performance eco-friendly barrier materials that could replace synthetic polymers whose composition and manufacturing processes present ecological challenges is growing. Polysaccharides are natural biopolymers already produced in large volumes for many industries including papermaking, textiles, and food production. Cellulose, starch, chitin, and their chemical derivatives, including carboxymethyl cellulose (CMC) and chitosan (CS) are among the highest volume, least expensive biopolymers produced. These polymers, however, are highly hydrophilic and do not possess adequate liquid barrier properties. Exceptional barrier behavior using these polymers has been achieved by combining them in polyelectrolyte complexation. Specifically, cationic CS and anionic CMC have been combined under high-shear homogenization to creat nanostructured particles that electrostatically coalesce during dehydration, forming a dense insoluble material. The current study demonstrates that this material is resistant to the penetration of grease (TAPPI T 559 cm-02, kit number 12), vegetable oil, and water. With the addition of rigid cellulose nanocrystals, the resulting materials exhibited improved mechanical and water vapor barrier properties. This work demonstrates that electrostatic complexation can be used to produce sustainable polysaccharide-based materials with unprecedented performance useful for replacing synthetics or higher cost alternatives in many high-volume applications including paper, food engineering, textiles, packaging, and construction.