The covalent functionalization of graphene oxide (GO) with chitosan (CS) is successfully accomplished via a facile amidation process. The CS-grafted GO (GO–CS) sheets consist of about 64 wt.% CS, which imparts them with a good aqueous solubility and biocompatibility. Additionally, the physicochemical properties of GO–CS are studied. As a novel nanocarrier, GO–CS is applied to load a water-insoluble anticancer drug, camptothecin (CPT), via π–π stacking and hydrophobic interactions. It is demonstrated that GO–CS possesses a superior loading capacity for CPT, and the GO–CS–CPT complexes show remarkably high cytotoxicity in HepG2 and HeLa cell lines compared to the pure drug. At the same time, GO–CS is also able to condense plasmid DNA into stable, nanosized complexes, and the resulting GO–CS/pDNA nanoparticles exhibit reasonable transfection efficiency in HeLa cells at certain nitrogen/phosphate ratios. Therefore, the GO–CS nanocarrier is able to load and deliver both anticancer drugs and genes.