In the title polymer, {[Co(C(14)H(10)N(2)O(4))(H(2)O)]·3.5H(2)O}(n), the Co(2+) ion is coordinated by three carboxyl-ate O atoms from two 5-[(pyridin-2-ylmeth-yl)amino]-isophthalate anions, two N atoms from a (pyridin-2-ylmeth-yl)amino group and an O atom from a water mol-ecule, furnishing a distorted CoO(4)N(2) octa-hedral geometry. Each anion acts as a μ(3)-bridge, linking cobalt ions into a two-dimensional layer parallel to (100). The asymmetric unit also contains three and a half solvent water mol-ecules, which could not be modeled. Therefore, the diffraction contribution of the solvent water mol-ecules was removed by the subroutine SQUEEZE in PLATON [Spek (2009). Acta Cryst. D65, 148-155]. The crystal structure is stabilized by O-H⋯O hydrogen bonds in which the coordinated water mol-ecule acts as donor and the carboxyl-ate O atoms as acceptors.