The effect of edge-functionalization on the competitive adsorption of a binary CO₂–CH₄ mixture in nanoporous carbons (NPCs) has been investigated for the first time by combining density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulation. Our results show that edge-functionalization has a more positive effect on the single-component adsorption of CO₂ than CH₄, therefore significantly enhancing the selectivity of CO₂ over CH₄, in the order of NH₂–NPC > COOH–NPC > OH–NPC > H–NPC > NPC at low pressure. The enhanced adsorption originates essentially from the effects of (1) the conducive environment with a large pore size and an effective accessible surface area, (2) the high electronegativity/electropositivity, (3) the strong adsorption energy, and (4) the large electrostatic contribution, due to the inductive effect/direct interaction of the embedded edge-functionalized groups. The larger difference from these effects results in the higher competitive adsorption advantage of CO₂ in the binary CO₂–CH₄ mixture. Temperature has a negative effect on the gas adsorption, but no obvious influence on the electrostatic contribution on selectivity. With the increase of pressure, the selectivity of CO₂ over CH₄ first decreases sharply and subsequently flattens out to a constant value. This work highlights the potential of edge-functionalized NPCs in competitive adsorption, capture, and separation for the binary CO₂–CH₄ mixture, and provides an effective and superior alternative strategy in the design and screening of adsorbent materials for carbon capture and storage.