This study focuses on development of a new adsorption technique by CO-activated chitosan. Carbon dioxide was utilized to form the functional chemical groups of chitosan on the adsorptions of anionic dyes, Brilliant Blue FCF and Congo Red, in the aqueous solution. CO-activated chitosan results in the dye adsorption significantly faster than that of chitosan in pure water. The adsorption capacities and removal efficiencies of the dye are increased by CO-activated chitosan. Furthermore, the dye adsorptions on CO-activated chitosan were investigated at various temperatures and initial dye concentrations in the aqueous solution. Interestingly, the high temperature adsorption provides the enhancement of adsorption capacities and removal efficiencies of the dye by the carbamate cross-linking of chitosan with CO. CO-activated chitosan was further characterized by Fourier transform infrared spectra, amino group ratio, zeta potential, and thermal gravimetric analysis. These characterizations can be used for understanding the unique adsorption of the dye on CO-activated chitosan. Carbon dioxide-activated chitosan in this work will provide an effective operation and a clean process of dye adsorption in wastewater treatment.