While natural biopolymers such as chitosan and cellulose have demonstrated huge potential in many important and rapidly growing applications, it is always challenging to create advanced functional biopolymer materials cost-effectively. Here, we report our advance in preparing biopolymer polyelectrolyte complexed materials based on chitosan and carboxymethyl cellulose (CMC) using a facile “dry”, thermomechanical method. Despite the high hydrophilicity of chitosan and CMC, the resulting (B) films showed much better dimensional stability and structural integrity in water, compared to chitosan-only (A) films. This is so despite inclusion of the CMC in the matrix increasing the surface hydrophilicity. We propose that our processing method led to the polyelectrolyte complexation between chitosan and CMC generating physical crosslinking points in the materials. These crosslinks stabilised the films in water although hydration disrupted hydrogen bonding between biopolymer chains leading to dramatically decreased tensile strength and increased elongation at break. Moreover, nanoparticles (montmorillonite and sepiolite) were more finely dispersed in the chitosan (A) matrix than in chitosan/CMC (B), suggesting competing interactions in this ternary system. The novel biopolymer polyelectrolyte complexed materials developed here, without chemical reactions, will be highly beneficial especially for biomedical applications requiring excellent biocompatibility, biosafety, biodegradability, antimicrobial and antifungal activity where exceptional hydrolytic stability during in-service use is also demanding (e.g. in implants, antimicrobial wound healing, tissue engineering scaffolds, drug delivery carriers).