Rare-earth niobate and tantalate materials are of considerable interest for use as phosphors, photocatalysts and ionic conductors. We successfully synthesized Ln₃MO₇ (Ln = Y, Ce, Er, Ho, Tm, Yb and Lu, M = Ta, Nb) nanophosphors by a hydrothermal method using the water-soluble Lindqvist ion polyoxometalates [HNb₆O₁₉]⁷⁻ and [Ta₆O₁₉]⁸⁻ as Nb and Ta sources. The Pawley refinements of these nanophosphors revealed that the Lu₃TaO₇ and Lu₃NbO₇ nanophosphors could be indexed in the cubic system with the space group Fmm, and that Y₃TaO₇ crystallizes in orthorhombic symmetry with the space group of C2221. These three solid compounds are nanoparticles with average particle sizes of 7.5, 5.9 and 4.0 nm for Lu₃TaO₇, Lu₃NbO₇ and Y₃TaO₇, respectively. The photoluminescence properties of Eu³⁺ doped Lu₃TaO₇, Lu₃NbO₇ and Y₃TaO₇ were studied, and Eu³⁺/Sr²⁺ co-doped Ln₃MO₇ (M = Ta, Nb) has an enhanced emission intensity compared to that of Ln₃MO₇:Eu³⁺ (M = Ta, Nb).