Treating dye-containing wastewater poses numerous challenges due to its high chemical complexity and its persistent nature. Thus, the present study aims to synthesize biochar derived from banana peel (BC) and its nanocomposites with copper oxide nanoparticles (CuO_x/BC_1-x) for the purpose of adsorptive removing Congo red (CR) dye from water. Several analytical methods were utilized to describe the physicochemical features of the CuO_x/BC_1-x nanocomposites. It was found that the crystallinity of the nanocomposites gradually improved, while the specific surface area and the surface electronegativity were reduced with increasing x value. The effects of x values (0–0.5), interaction time (10–120 min), adsorbent dose (0.01–0.05 g), initial CR concentration (20–200 mg/L), and the solution temperature (20–60 °C) were evaluated on CR removal. The obtained results revealed that the CuO_0.5/BC_0.5 nanocomposite showed the highest adsorption efficiency with a maximum adsorption capacity of 233.6 mgg⁻¹. Analysis of the equilibrium experimental data revealed that the Langmuir and the pseudo-2nd-order models were the most proper to describe the current adsorption process. Moreover, the thermodynamics studies demonstrated that the adsorption process was spontaneous, endothermic, and random.