The crystal structure of the new triphosphate K2Na3P3O10: 5% Eu3+ (monoclinic, space group C2/c, unit cell parameters: a = 9.8695(3) Å; b = 5.6171(2) Å; c = 18.6404(6) Å; β = 96.0947(2) °) has been solved from single crystal data. The structure is closely related to that of the undoped material K2Na3P3O10 and indicates the presence of rare earth ions in all alkaline crystallographic sites. Eu3+ doped K2Na3P3O10 triphosphates were synthesized as polycrystalline samples by solid state reaction and characterized by powder X-ray diffraction and infrared spectroscopy. The optical properties were recorded in the VUV-visible range. Broad and intense bands are observed in the 150–250 nm range, corresponding to the absorption by the host matrix and Eu-O charge transfer state (CTS). The numbering of the Eu3+5D0 → 7FJ (J = 0–4) emission transitions observed in the visible domain is in good agreement with the crystal structure data and is discussed according to the symmetry of the different sites occupied by the lanthanide ions in the triphosphate lattice.