Polycitric acid was conjugated onto the surface of carbon nanotubes (CNTs), and hybrid nanomaterials containing CNT axis and polycitric acid branches that were highly soluble in aqueous solutions were synthesized. In this work, pristine MWCNTs were opened and functionalized through treatment with acid, then polycitric acid was covalently grafted onto their surface by the “grafting from” approach based on polycondensation of citric acid in the melting state. The chemical structure, morphology, thermal properties and size of MWCNT-g-PCA hybrid materials were investigated by IR, 13C NMR, 1H NMR, TEM, TGA, DSC and DLS. To investigate the biocompatibility of the synthesized hybrid nanomaterials and their potential applications for future nanomedicine, short-term in vitro cytotoxicity and hemocompatibility tests were conducted on HT1080 cell line (human fibrosarcoma). Based on the results of the in vitro cytotoxicity tests and hemolysis assay, no adverse effect was observed on the HT1080 cell and also on red blood cells up to 1 mg/mL concentration. It appeared that the changes in the conformation, shape and dispersity of CNTs, induced by the conjugated hyperbranched polymers, were the main factors that affected the toxicity of CNTs and also their interaction with the cell membranes. Interestingly, the results of the cytotoxicity tests were in agreement with our previous work, carbon nanotubes-graft-polyglycerol, proving that hyperbranched polymers conjugated onto the surface of CNTs dominated their physiochemical properties and therefore their interactions with the cell membranes.