The global climate change induced by greenhouse gases has stimulated active research for developing efficient strategies to mitigate CO₂ emission. In the present study, we prepared a series of polyamine/metal–organic framework (MOF) composites as highly selective CO₂ adsorbents from a CO₂/N₂ mixture, which is relevant to CO₂ capture in flue gas. We show that loading polyethyleneimine (PEI) into MIL-101(Cr) frameworks can significantly enhance the selective CO₂ adsorption capacity at low pressure and ambient temperature. Further, the comparative study reveals that both the particle size of the MOF and the molecular-weight of PEI play an important role in the CO₂ capture ability. Regarding the particle size, smaller MIL-101(Cr) particles can facilitate the loading of PEI into the inner pores and result in lower surface area/pore volume. Thus, the resulting PEI/MIL-101(Cr) composites possess lower CO₂ adsorption capacity, but are compensated by higher selectivity of CO₂ over N₂. On the other hand, lower molecular-weight linear PEI could readily diffuse into the inner pores and effectively block the N₂ adsorption. As a result, the as-prepared A-PEI-300 sample in this work exhibits an excellent CO₂ uptake of 3.6 mmol g⁻¹ and ultrahigh CO₂/N₂ selectivity at 0.15 bar and 25 °C. In contrast, the higher molecular-weight branched PEI is advantageous at elevated temperature, since the composites can retain high CO₂ adsorption capacity owing to the large amount of primary amine groups. Overall, polyamine/MOF composites are shown to be good candidate adsorbents for CO₂ capture from flue gas. To achieve the optimal CO₂ capture ability, comprehensive optimization of the polyamine and MOF structures should be performed.