Polyethylenimine-functionalized porous chitosan (PEICS) beads were prepared and their CO2 adsorption performance was evaluated. The CO2 adsorption capacity of PEICS was dependent upon both the amine content and surface area of the functionalized beads. PEICS showed a CO2 adsorption capacity of 2.3 mmol/g at 313 K and 15 kPa of CO2 in the absence of water vapor that considerably increased to 3.6 mmol/g in the presence of water vapor. To rationalize this phenomenon, the CO2 adsorption mechanisms in the absence and presence of water vapor were investigated by diffuse reflectance infrared Fourier transform spectroscopy. The results indicated that the mechanism of CO2 adsorption onto PEICS, in both the absence and presence of water vapor, involved the formation of carbamate. Therefore, the higher CO2 adsorption capacity in the presence of water vapor was attributed to the increased accessibility to amino groups of PEICS, owing to swelling of the polyethylenimine chain and/or chitosan framework upon adsorption of water. The herein reported chitosan-based material displays high CO2 adsorption capacity as well as excellent regenerability and, thereby, shows potential as an adsorbent for CO2 capture.