Highly porous activated carbons were prepared from melamine-modified phenol–formaldehyde resins via steam activation at different activation temperatures (700–950 °C) and then used for CO2 capture at atmospheric pressure. The porosity and surface properties of the activated carbons and their CO2 adsorption capabilities were studied and compared with those derived from common phenol–formaldehyde resin. An adsorption capacity for CO2 of 6.71 mol kg−1 (29.5 wt.%) was attained at 0 °C and 1 atm and decreased significantly with increasing operating temperature. The prepared activated carbons could be well applied in temperature-swing adsorption processes. Of the three isotherm equations (Langmuir, Freundlich, and Redlich–Peterson), the adsorption of CO2 on activated carbons could only be satisfactorily described using the three-parameter Redlich–Peterson equation.