Three-dimensional (3D) crumpled graphene-based porous adsorbents with highly interconnected networks were synthesized through a facile one-step physical activation method using reduced graphene oxide (RGO) as precursor, for separation of carbon dioxide (CO₂) from postcombustion flue gas mixtures. Due to their large surface area (above 1300 m² g^–1), high pore volume (over 1 cm³ g^–1), and well-defined bimodal microporous–mesoporous structure, these hierarchical porous graphene-based materials demonstrate rapid, stable, reversible, and high CO₂ uptake of 2.45 mmol g^–1 at 25 °C and 1 bar. Importantly, the CO₂ over nitrogen (N₂) adsorption selectivity is among the best at partial pressures relevant to capturing CO₂ emanating from postcombustion power plants fueled with either coal or natural gas. Moreover, the isosteric heat of adsorption of only −27.42 kJ mol^–1 at zero coverage reflects the ease of adsorbent regeneration with significantly low energy consumption, which in turn might reduce the costs of carbon capture and sequestration.