In the title compound, poly[sodium-mu(4)-3,5-dicarboxybenzenesulfonato-kappa4O:O':O":O"'-mu(2)-urea-kappa2O:N] monohydrate], [[Na(C8H5O7S)(CH4N2O)] x H2O]n, the organic anions are arranged almost vertically within (001) monolayers, with the sulfonate and carboxylic acid groups pointing into the interlayer region. The inversion-related aromatic rings of the anions inside the layers are arrayed via offset face-to-face interactions into molecular stacks along the crystallographic a axis. The 'up' and 'down' arrangement of the aromatic portions makes both faces of the layers ionic and hydrophilic, whereas the interiors of the layers are primarily hydrophobic. The interleaving of the anions is such that the carboxylic acid groups are oriented more toward the interior than are the sulfonate groups. The aromatic rings in neighbouring layers are arranged in a herring-bone fashion. The coordination sphere of the Na+ ions contains two sulfonate and two carboxylic acid O atoms, from a total of four different acid anions belonging to two neighbouring anionic monolayers. The urea molecules are positioned between translation-related anionic stacks inside the (001) layers, serving a triple function, viz. they fill in the large meshes (empty cavities) formed within the anionic-cationic network, and they provide additional Na+ coordination and hydrogen-bond sites.