UiO-66-NH₂, a zirconium-based functional metal–organic framework (MOF), was postsynthetically modified via Schiff base reaction between aldehyde groups in glutaraldehyde and amino groups in UiO-66-NH₂ and CO₂-preabsorbed polyethyleneimine (PEI). The resulting PEI-modified MOFs, abbreviated as PEI^C@UiO, were characterized with ¹H NMR, Fourier transform infrared, powder X-ray diffraction, Brunauer–Emmett–Teller, scanning electron microscopy, and thermogravimetric analysis and evaluated as CO₂ adsorbents. In comparison with pristine UiO-66-NH₂, the PEI^C@UiO adsorbents have reduced specific surface area (7–150 m²/g) but maintained the same crystal structure. Particularly, the PEI^C96@UiO adsorbent exhibited significantly improved CO₂/N₂ adsorption selectivity (48 vs 25) and higher CO₂ adsorption capacity (3.2 vs 2.7 mmol/g). The adsorbent also displayed moderate desorption energy (68 kJ/mol CO₂), superior moisture endurance, and recyclability, which are very desirable for practical applications.