A new phosphate K2(VO)2P3O9(OH)3•1.125H2O has been synthesized hydrothermally in a sealed quartz glass tube under autogeneous pressure by slowly cooling from 450 °C a mixture of K4V2O7, V2O3, and 7.25 M H3PO4 and characterized by single-crystal X-ray diffraction and thermogravimetric analysis. The title compound crystallizes in the triclinic space group [formula omited] with a = 6.404 (2) Å, b = 8.9512 (9) Å, c = 12.070 (1) Å, α = 106.234 (9)°, β = 98.61 (2)°, γ = 103.33 (2)°, V = 629.2 (2) Å3, Z = 2, R = 0.025, and Rw = 0.028 for 1896 unique reflections with I > 2.5σ(I). The structure consists of slabs of corner-sharing vanadium oxide octahedra and hydrogen phosphate groups lying in the ab plane with atom K(1) between the slabs. Within a slab there are channels where atom K(2) and a water of hydration are located. Each channel results from the stacking of the edges of four octahedra and four tetrahedra.