In this study, polyethylenimine (PEI) modified Lagerstroemia speciosa leaves powder (PEI-LS) as low-cost and ecofriendly biosorbent was efficiently applied for gold recovery from acidic solution. Our experimental data show that PEI-LS is effective in complete recovery of gold at pH 1, compared to the raw L. speciosa leaves powder (raw LS) biomass. The adsorption kinetic results show gold adsorption reaches equilibrium in 6 h and follows a pseudo-second order kinetic. The adsorption isotherm data were best explained by Freundlich model and the monolayer adsorption capacity of PEI-LS for gold are in the range 286-313 mg/g. Thermodynamic parameters suggest that gold adsorption on PEI-LS was spontaneous and endothermic in nature. The developed PEI-LS was reusable up to 4 cycles with 98% gold recovery. Further, the calculated separation factor values suggest the PEI-LS shows high selectivity for low concentration of gold over the other co-existing metal ions in binary mixtures. Various instrumental characterization of raw IS and PEI-LS confirms the successful grafting of PEI on biomass surface. Surface analytical techniques like X-ray photoelectron spectroscopic (XPS) analysis confirms the adsorption-reduction mechanism of gold ions on PEI-LS leading to the formation of zero-valent gold on the biosorbent surface. The XPS was also used to characterize the PEI-LS surface functional groups (N1s) responsible for gold adsorption-reduction mechanism. The PEI-LS was successfully applied for quantitative gold recovery (>= 95%) from an acidic solution of electronic scrap, gold refining sludge and wastewater.