A new metal-organic framework compound, poly[[μ7-dihydrogen (4,5-dicyano-1,2-phenylene)diphosphonato](oxonium)caesium], [Cs(C8H4N2O6P2)(H3O)] n (I), based on Cs+ and the organic linker 4,5-dicyano-1,2-phenylene)bis(phosphonic acid, (H4cpp), containing two distinct coordinating functional groups, has been prepared by a simple diffusion method and its crystal structure is reported. The coordination polymeric structure is based on a CsO8N2 complex unit comprising a monodentate hydronium cation, seven O-atom donors from two phosphonium groups of the (H2cpp)2- ligand, and two N-atom donors from bridging cyano groups. The high level of connectivity from both the metal cation and the organic linker allow the formation of a compact and dense three-dimensional network without any crystallization solvent. Topologically (I) is a seven-connected uninodal network with an overall Schäfli symbol of {417.64}. Metal cations form an undulating inorganic layer, which is linked by strong and highly directional O – H⋯O hydrogen-bonding interactions. These metallic layers are, in turn, connected by the organic ligands along the [010] direction to form the overall three-dimensional framework structure.