The mechanism of the anticorrosion effects of chitosan grafted with poly-aspartic acid (CTS-PAA) against mild steel in 1 M HCl has been provided using combined electrochemical and theoretical approaches. The successful grafting of chitosan (CTS) with poly-aspartic acid (PAA) was verified with intensive Furrier-transformed infrared spectroscopy (FTIR).   In the studied system, a minimal corrosion inhibition efficiency of about 60 % was obtained with the unmodified CTS whereas CTS-PAA inhibition efficiency reaching a maximum number of 98.9 % at 250 ppm concentration was obtained. Polarization studies show that the inhibitors acted on both the anodic and cathodic currents curves indicating a mixed inhibition protection mechanism while the electrochemical impedance data show that the protection was afforded by interfacial adsorption of the inhibitor species on the steel surface. The adsorption process fitted to Langmuir adsorption isotherm. The proposed inhibition mechanism linked the enhanced inhibition performance of CTS-PAA to the extension of the adsorption sites by the grafting agents which was confirmed with X-ray photoelectron spectroscopy (XPS) results. The experimental data were correctly complemented with density functional theory (DFT)/molecular dynamic simulation (MDS) data and the binding energy of the modified polymer is over two magnitudes higher than the individual molecules affirming the experimental findings.