A new Mo5+ phosphate, γ-CsMo2P3O13, was discovered and its crystal structure was determined from single-crystal X-ray diffraction data. γ-CsMo2P3O13 crystallizes in the triclinic space group P1 with a = 6.342(1), b = 9.676(1), c = 10.0349(8) Å, α = 83.083(7), β = 97.42(1), γ = 108.30(1)°, V = 577.7(2) Å3, Z = 2, R = 0.034, Rw = 0.042 for 2927 independent reflections with I > 2.5 σ(I). The structure consists of three different tunnels of which the large and medium ones are occupied by cesium cations. The framework is built up from MoO6 octahedra, PO4 tetrahedra, and P2O7 groups. The γ-CsMo2P3O13 structure is closely related to that of the β-form.