[en] It is now recognized that adsorption using low-cost adsorbents is an effective and economic method for water decontamination. Chitosan is derived by deacetylation of the naturally occurring biopolymer chitin which is the second most abundant polysaccharide in the world after cellulose. In this study, the capacity of the chitosan nanoparticles was studied for the removal of the anionic Congo red from wastewater. The effect of various parameters, such as the initial pH value in the range of 2-4, initial dye concentration (3-50 mg/L), adsorbent dosage (0.1-1.5 g), contact time (5-40 min) and temperature (20-45 °C) on adsorption process was investigated. The results indicated that chitosan nanoparticles under optimal conditions (pH=7, 5 mg/L^-1 concentration of adsorbent, contact time of 15 minutes and a temperature of 30 °C) are able to remove 99.96% of Congo red from aqueous solutions. Hence, nano-chitosan can be used as an eco-friendly adsorbent for the removal of Congo red from wastewater in terms of lower required time and higher adsorption capacity.