The title compound, [Co(C15H11N3)(H2O){Pt(CN)4}] n , is a one-dimensional coordination polymer formed under hydro-thermal reaction conditions. The Co(II) site has sixfold coordination with a distorted octa-hedral geometry, while the Pt(II) ion is coordinated by four cyanide groups in an almost regular square-planar geometry. The compound contains twofold rotation symmetry about its Co(II) ion, the water molecule and the terpyridine ligand, and the Pt(II) atom resides on an inversion center. trans-Bridging by the tetra-cyanidoplatinate(II) anions links the Co(II) cations, forming chains parallel to [-101]. Additionally, each Co(II) atom is coordin-ated by one water mol-ecule and one tridentate 2,2':6',2''-terpyridine ligand. O-H⋯N hydrogen-bonding inter-actions are found between adjacent chains and help to consolidate the crystal packing. In addition, relatively weak π-π stacking inter-actions exist between the terpyridine ligands of adjacent chains [inter-planar distance = 3.464 (7) Å]. No Pt⋯Pt inter-actions are observed in the structure.