A novel chemical functionalization of guar gum (GG) by benzoic acid (BA) via nucleophilic substitution reaction in aqueous solution has been reported. BA moieties are chosen due to coordination chemistry of carboxylic acid moieties, hydrophobicity and intermolecular interaction of aromatic rings. The presence of conjugated BA on guar gum-benzoic acid (GG-BA) with grafting density of 5.5% is confirmed by 1H NMR. Amorphous GG-BA with irregular morphology has been studied by UV–Vis, FTIR, XRD, SEM, TEM, TGA, computational chemistry and contact angle measurement. GG-BA in a concentration range from 0 to 4000 μg mL−1 has good biocompatibility to mouse embryonic fibroblasts (MEF), human mammary epithelial cells (MCF-10A) after 48 and 72 h of treatment using WST-1 assay. GG-BA shows great potential for the development of biomaterials such as bioadhesives, hydrogels, and coacervates. [Display omitted] • 5.5 % grafting density of benzoic acid on guar gum. • A mild, environmentally friendly reaction condition in aqueous solution is established. • Guar gum benzoic acid is confirmed by a wide range of techniques. • Electronic structures of GG-BA are computationally modeled and studied to show good agreement with UV-Vis spectra. • Guar gum benzoic acid in a concentration range from 0 to 4000 μg mL-1shows good biocompatibility to MEF and MCF-10A cells. [ABSTRACT FROM AUTHOR]