The CoII atom in the title complex, [Co(SO4)(C12H8N2)(H2O)3] (or C12H14CoN2O7S), is octa-hedrally coordinated within a cis-N2O4 donor set defined by the chelating N-donors of the 1,10-phenanthroline ligand, sulfate-O and three aqua-O atoms, the latter occupying an octa-hedral face. In the crystal, supra-molecular layers lying parallel to (110) are sustained by aqua-O-H⋯O(sulfate) hydrogen bonding. The layers stack along the c-axis direction with the closest directional inter-action between them being a weak phenanthroline-C-H⋯O(sulfate) contact. There are four significant types of contact contributing to the calculated Hirshfeld surface: at 44.5%, the major contribution comes from O-H⋯O contacts followed by H⋯H (28.6%), H⋯C/C⋯H (19.5%) and C⋯C (5.7%) contacts. The dominance of the electrostatic potential force in the mol-ecular packing is also evident in the calculated energy frameworks. The title complex is isostructural with its manganese, zinc and cadmium containing analogues and isomeric with its mer-tri-aqua analogue.