Porous activated carbons are considered to be promising CO2 adsorbents due to their high specific surface area, high chemical stability, and tailorable surface properties. However, their low CO2 capture capacity and inferior CO2/N-2 selectivity have hindered their application. Here, we describe novel fishnet-like, polybenzoxazine-based porous carbons (PBZCs) prepared by a single-step monomer thermal curing, carbonization, and activation process. The PBZCs exhibit an ultrahigh CO2 uptake capacity of 8.44 mmol g(-1) and a superior CO2/N-2 IAST selectivity of 56 (at 273 K, 1 bar). Such excellent CO2 adsorption performance may to some extent be ascribed to a high specific surface area and a large ultramicropore volume. However, the results reveal that the CO2 capture capacity is not solely associated with porosity. It may also be attributable to the abundant hydroxyl groups of the PBZCs, which may form hydrogen bonds with CO2 molecules. The role of the oxygen functionalities of porous carbon for CO2 capture was further demonstrated through theoretical calculation combined with experimental analysis. Hydrogen bonding lowers the binding energy between the carbon framework and CO2 molecules, which greatly facilitates CO2 adsorption. Furthermore, the novel fishnet-like structure can anchor CO2 molecules effectively and selectively. These PBZC carbons are potentially promising CO2 adsorbents.