The crystal structure of Cs₂CuP₂O₇ [monoclinic, Cc, a=7.460(6), b=12.973(10), c=9.980(8) Å, β=111.95(2)°, V=895.8(12) ų] prepared by solid-state reactions is based upon open framework formed by corner sharing between CuO₄ distorted squares and P₂O₇ groups. The framework is porous and has a very low framework density of 13.4 Cu+P atoms per 1 nm³. Cs⁺ cations reside in large framework cavities. The heteropolyhedral network in the title compound is based upon three-dimensional (3;4)-connected net that has a three-membered CuP₂ ring as its elementary unit. In terms of reticular chemistry, this net should be considered as an augmented diamond (dia) net. The Cu-P net can be obtained from the latter by the replacement of its nodes by the CuP₂ triangles. This replacement is strongly non-centrosymmetric, since all CuP₂ triangles are oriented along the same direction, which provides a crystal chemical explanation for the absence of a symmetry centre in the structure. Cs₂CuP₂O₇ is the first compound in the A₂CuP₂O₇ family (A=alkaline metal), which is based upon three-dimensional copper pyrophosphate framework.