The crystal structure of a new potassium tantalum(V) cyclotetrasilicate, KTaSi2O7(K2Ta2Si4O14), has been determined by single crystal X-ray diffraction. KTaSi2O7crystallizes in the tetragonal system, space groupP4/mbmwitha= 8.735(2) andc= 7.999(2) Å,V= 610.3(2) Å3,Z= 4,dx= 4.225 g/cm3, λ (MoKα) = 0.71073 Å, μ = 190.7 cm−1,F′ (000) = 704,T= 293 K,R= 0.0421,Rw= 0.0789 for 451 unique reflections (Rint= 0.0227) withF2 0> 0. The framework structure of KTaSi2O7is built up from chains of corner-sharing TaO6octahedra running parallel to the fourfold axes and linked together by four-membered Si4O12single units. AC impedance measurements on KTaSi2O7indicate low ionic conductivities, 5.6 × 10−9and 3.0 × 10−7Ω · cm−1at 333 and 546°C, respectively. An activation energy of 0.83 ± 0.02 eV is observed.