The aim of the work was to investigate the biosorption potential of eggshell for the removal of Congo red (CR) from model dye solutions and synthetic wastewater with the addition of dye . Batch adsorption experiments were performed and various factors influencing the biosorption process were tested: Biosorbent concentration (1-15 \(g L^{-1}\)), contact time (1-360 min), initial dye concentration (10-100 \(mg L^{-1}\)) and temperature (15, 25, 35, 45 °C). The amount of adsorbed dye per gram of biosorbent decreases as the concentration of biosorbent increases, while the percentage of dye removal increases. The process of biosorption of CR on the eggshell is rapid in the first 60 minutes of the process and then begins to slow down until equilibrium is reached after 240 minutes. The process is dependent on the initial concentration of the dye, with an increase in the initial concentration of CR leading to an increase in the amount of dye adsorbed per gram of biosorbent (from 0.98 \(mg g^{-1}\) to 4.68 \(mg g^{-1}\)), with a simultaneous decrease in the percentage of dye removal (from 94.10% to 44.32%). The increase in temperature had a positive effect on the biosorption process, and the increase in temperature led to a continuous increase in the percentage of dye removal. The Langmuir model describes the process of CR biosorption on the eggshell slightly better than the Freundlich model, while the kinetics of biosorption under the applied experimental conditions is better described by the pseudo-second-order model than by the pseudo-first-order model. Higher dye removal efficiency and faster establishment of the equilibrium state were achieved when the biosorption process was carried out in synthetic wastewater with the addition of dye compared to the process in a model dye solution.