Nanostructured polyaniline and its composites with pine cone-based activated carbon were synthesized by the oxidative polymerization of aniline in an aqueous solution of sulfuric acid and in the presence and absence of appropriate surfactant. The prepared activated carbon (AC), AC/polyaniline nanofiber composite (AC–PANI-F) and AC/polyaniline nanosphere composites (AC–PANI-S) were characterized by an N₂ adsorption isotherm at 77 K, elemental analysis, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and acid–base Boehm titration. The equilibrium adsorption of CO₂ on AC and nanocomposites were experimentally investigated via a volumetric technique at a temperature range of 298–318 K and pressures up to 16 bar. The CO₂ adsorption capacity was remarkably increased from 1.91 mmol g⁻¹ for AC to 2.69 mmol g⁻¹ for AC–PANI-F and 3.16 mmol g⁻¹ for AC–PANI-S at 25 °C and 1 bar. The Sips isotherm presented a perfect fit to the CO₂ adsorption data on adsorbents. The relatively fast kinetic adsorption of CO₂ on the nanocomposites reflected the presence of more easily accessible adsorption sites toward CO₂ for nanocomposites than AC. The isosteric heat of adsorption at relatively low adsorbate coverage was 55 kJ mol⁻¹ for AC–PANI-F and 52 kJ mol⁻¹ for AC–PANI-S, indicating the physico-chemical characteristics and heterogeneity of the adsorbents surface. The selectivity of CO₂ over N₂ by AC, AC–PANI-F and AC–PANI-S (CO₂ : N₂ = 15 : 85, 298 K, 1 bar), predicted by the ideal adsorbed solution theory (IAST) model, achieved 1.53, 18.97 and 6.1, respectively. The results indicate that activated carbon/PANI nanocomposites can be used as an effective adsorbent for capture of CO₂ from flue gas.