Two new ternary iron(III) diphosphates, CsFeP2O7 and RbFeP2O7, have been prepared and the structure of the Cs compound was determined from single-crystal X-ray diffraction data. Base on powder X-ray diffraction RbFeP2O7 adopts the same structure type as that of the Cs compound. CsFeP2O7 crystallizes in the monoclinic space group P21/c with a = 7.684(3), b = 9.937(3), c = 8.378(3) Å, β = 104.84(3)°, V = 618.4(4) Å3, Z = 4, and R = 0.048. The structure consists of intersecting tunnels running along the  and  directions, and the Cs+ cations are located at the intersection of these tunnels. The framework is built up from corner-sharing FeO6 octahedra and P2O7 groups. CsFeP2O7 is isostructural with KFeP2O7. Variable-temperature powder magnetic susceptibility data exhibit an antiferromagnetic transition at TN ∼ 25 K.