Single crystals of K2Ca3Si3O10 have been obtained in a multi-anvil high-pressure synthesis experiment performed at 10 GPa and 1000 °C. The compound adopts the monoclinic space group C2/c with four formula units per cell and the following lattice parameters (at 25 °C): a = 10.3539(19) Å, b = 10.6013(16) Å, c = 9.8221(19) Å, β = 118.079(13)°, V = 951.2(3) Å3. The crystal structure was determined from single-crystal X-ray diffraction data using direct methods (Mo-Kα radiation, 2𝛉max = 59.53°, Rint = 5.90 %) and refined to R(|F|) = 5.64 % using 1042 observed reflections with I > 2σ(I). The structure belongs to the group of oligosilicates consisting of [Si3O10] groups. The trimers of the anion complex are located in layers parallel to (001) at about z ≈ ¼ and ¾, respectively. Ca(1)-octahedra provide linkage between (1) the Si3O10 groups of a single layer by corner sharing of the equatorial oxygen atoms of the terminal tetrahedra and (2) the trimers belonging to adjacent sheets by corner sharing of the apical O-atoms. To an upper limit of 3.3 Å, the remaining two crystallographically independent non-tetrahedral cation sites Ca(2) and K(1) are coordinated by 8 and 10 oxygen atoms, respectively. From a topological point of view the crystal structure of K2Ca3Si3O10 can be classified as a new type of mixed tetrahedral-octahedral framework. The thermal expansion of K2Ca3Si3O10 has been determined in the temperature range between 25 and 750 °C. The temperature dependence of the cell volume can be described with a second-order polynomial: V(T) = 0.00002(3)T2 + 0.026(2)T + 949.36(34). Structural investigations were completed by Raman spectroscopic studies. The assignment of the bands to certain vibrational species was aided by density functional theory (DFT) calculations.