Recrystallization of Cs3P7 from liquid NH3 yields the triammoniate Cs3P7·3 NH3, which loses the weakly bound NH3 of crystallization below 253 K. A low-temperature crystal-structure analysis shows that Cs3P7· NH3 consists of a framework of heptaphosphanortricyclane anions Pmath image and Cs+ cations with NH3 molecules completing the coordination of the cations. The framework is built from Cs3P7 layers connected by only few Cs…P interactions, the interlayer gap being filled by a two-dimensional network of NH3. The Cs7P7 part of the structure completes a family of alkali-metal-polyphosphide substructures which range from math image[RbP7]2− or math image[CsP11]2− chains over math image[Cs2Pn] layers (n = 7, 11) to now math image[Cs3P7] frameworks.