Polymers obtained from renewable sources, generally biodegradable under normal environmental conditions, can be a viable solution to the waste disposal of plastic food packaging materials. However in order to compete with synthetic plastics, they should have comparable mechanical and/or barrier properties. This is especially difficult to achieve with respect to moisture barrier properties, because of the hydrophilic nature of most biopolymers which is in contrast with the hydrophobic nature of most synthetic polymers used in food packaging. Chitosan, a polysaccharide obtained by deacetylation of chitin, is one of the most interesting biopolymers obtained from natural sources. In addition to the biocompatibility characteristics, chitosan bacterio- and fungi-static properties are very useful for food preservation. As for other biopolymers, many strategies have been explored to improve the barrier and mechanical properties of chitosan-based films. These include the addition of other biodegradable aliphatic polyesters and, recently, the addition of inorganic nanoparticles including graphene. In this work, we report on the role of borate ions, originated from hydrolysis of sodium tetraborate decahydrate (borax), in the crosslinking of chitosan (CS) and graphene oxide (GO) through a solution casting approach to produce high barrier composite films. CS/GO films containing 10wt% borax and 1wt% GO exhibit two fold increments in tensile strength as compared to that of pristine CS. Also, the oxygen permeability of borate-crosslinked CS/GO composite film shows a one order of magnitude decrease with respect to the pristine chitosan. Solid-state 11B NMR spectra indicated that the borate orthoester formed between borate ions and GO or CS is responsible for the significant enhancement in both mechanical and barrier properties. These crosslinked graphene-based chitosan films offer new opportunities for a broad range of applications including packaging.