Polyethyleneimine functionalized graphene oxide (PEI-GO) was prepared and characterized for its molecular structure and morphology. The PEI-GO is, for the first time, reported as an inhibitor against copper (Cu) corrosion in 0.5 M HCl using electrochemical studies. PEI-GO acts as a promising corrosion inhibitor for Cu with 92.24% efficiency at 100 mgL-1. The PDP studies reveal a mixed type of inhibitor behavior of PEI-GO with cathodic prevalence. Cyclic voltammetry revealed the inhibition influence of PEI-GO on the redox process of Cu corrosion. SEM and FTIR-ATR studies showed the creation of a protective layer on the Cu substrate. DFT studies revealed that the PEIGO exhibits better reactivity compared to parent PEI, and Monte Carlo simulations showed higher adsorption energy for PEI-GO on the Cu surface compared to the PEI.