Noncovalent functionalization of antibacterial chitosan on chemically reduced graphene oxide (RGO) provides both dispersion stability and antibacterial activity for RGO, which is an insoluble and nonbioactive carbon nanomaterial. In situ chemical reduction of graphene oxide (GO) in the presence of chitosan under acidic conditions produces a stable dispersion of RGO/chitosan assemblies. The prepared RGO/chitosan assemblies show enhanced antibacterial activity for Escherichia coli JM109 compared with GO, suggesting that noncovalent chemistry can minimize any detrimental effects on the bioactivity, which is one of the critical features for biofunctionalization of carbon nanomaterials such as graphene.