The crystal structure of the title compound, [Co(CO3)(C12H8N2)2]NO3·H2O, consists of CoIII complex cations, nitrate anions and uncoordinated water molecules. The CoIII cation is chelated by a carbonate anion and two phenanthroline ligands in a distorted octahedral coordination geometry. A three-dimensional supramolecular structure is formed by O—H⋯O and C—H⋯O hydrogen bonding, C—H⋯π and aromatic π–π stacking [centroid–centroid distance = 3.995 (1)Å] interactions