The polymeric title compound, {[Co(C(15)H(9)NO(4)S)(H(2)O)(3)]·H(2)O}(n), consists of chains along [001] made up from Co(2+) ions bridged by 10-methyl-phenothia-zine-3,7-dicarboxyl-ate anions. The Co(2+) ion, coordinated by three O atoms from two different carboxyl-ate groups and three water mol-ecules, displays a distorted octa-hedral environment. In the crystal, π-π inter-chain inter-actions, with centroid-centroid distances of 3.656 (2) and 3.669 (2) Å between the benzene rings of the ligands, assemble the chains into sheets parallel to (100). O-H⋯O hydrogen-bonding inter-actions between the coordinating water mol-ecules and carboxyl-ate O atoms link the sheets into a three-dimensional network.