In the current study, new zinc oxide/activated carbon (ZnO/AC) nanocomposites were successfully synthesized, using Pontederia crassipes leaves as a carbon source for synthesis of activated carbon. In addition, the extract of Pontederia crassipes leaves was obtained as reducing and capping agent in the synthesis of ZnO. The synthesized materials were investigated using various characterization techniques including XRD, FTIR, FE-SEM, EDX, BET-BJH, TEM and Raman, which proved the successful synthesis. The synthesized samples were applied as new nanoadsorbents for the uptake of Malachite Green (MG) and Congo Red (CR) dyes from aquatic media. The thermodynamic, adsorption isotherm and kinetic studies were conducted adopting the batch experiment method. The results showed that the optimum efficiency of removal of both dyes was achieved with ZnO/AC (10%) nanocomposite, which was found to be 98.87% for CR dye and 99.23% for MG dye. The thermodynamic analysis exhibited that adsorption process for both dyes is spontaneous. In addition, the results exhibited that the removal of CR onto ZnO/AC(10%) was exothermic, while that of MG was an endothermic. In the case of adsorption isotherms, the Freundlich model showed the best fit for both dyes with a high correlation coefficient (R2) of 0.99, suggesting heterogeneous surface of the synthesized nanoparticles. The kinetic analysis indicated that both dyes followed a pseudo-second-order kinetic model.