The asymmetric unit of the mononuclear cobalt complex, [Co(C11H13O2)(2)(C6H6N2O)(2)(H2O)(2)]center dot 2H(2)O, contains one half of the complex molecule, one coordinating and one non-coordinating water molecule, one 4-tert-butylbenzoate (TBB) ligand and one nicotinamide (NA) ligand; the Co atom lies on an inversion centre. All ligands coordinating to the Co atom are monodentate. The four nearest O atoms around the Co atom form a slightly distorted square-planar arrangement, with the distorted octahedral coordination completed by the two pyridine N atoms of the NA ligands at distances of 2.1638 (11) angstrom. The coordinating water molecules are hydrogen bonded to the carboxyl O atoms [O center dot center dot center dot O = 2.6230 (17) angstrom], enclosing an S(6) hydrogenbonding motif, while intermolecular O-H center dot center dot center dot O hydrogen bonds link two of the non-coordinating water molecules to the coordinating water molecules and NA anions. The dihedral angle between the planar carboxylate group and the adjacent benzene ring is 29.09 (10)degrees, while the benzene and pyridine rings are oriented at a dihedral angle of 88.53 (4)degrees. In the crystal, O-H center dot center dot center dot O and N-H center dot center dot center dot O hydrogen bonds link the molecules, enclosing R-2(2)(8), R-2(2)(10) and R-4(4)(12) ring motifs, forming layers parallel to (001). The C and H atoms of the tert-butyl group of the TBB ligand are disordered over two sets of sites with an occupancy ratio of 0.631 (5):0.369 (5).