Carbon dioxide adsorbents are provided. The carbon dioxide adsorbents include a polymeric amine and a porous support on which the polymeric amine is supported. the polymeric amine consists of a polymer skeleton containing nitrogen atoms and branched chains bonded to the nitrogen atoms of the polymer skeleton. Each of the branched chains contains at least one nitrogen atom, the polymeric amine is modified by substitution of at least one of the nitrogen atoms of the polymer skeleton or the branched chains with a hydroxyl group-containing carbon chain.