High-pressure high-temperature behavior of metastable NaYF₄ fluorite (Fm₃- m, Z=4), with the Na and Y atoms randomly distributed in the cationic sublattice, is studied with synchrotron angle-dispersive powder X-ray diffraction in diamond anvil (DAC) and large-volume Paris–Edinburgh cells and synthesis in a multi-anvil apparatus. The onset of a pressure-induced phase transition at room temperature takes place above 10 GPa as observed in DACs loaded with different hydrostatic and non-hydrostatic pressure media (nitrogen, paraffin oil, or ethanol:methanol media). In situ powder X-ray diffraction measurements in the Paris–Edinburgh cell and syntheses using the multi-anvil apparatus at high pressures and high temperatures show that the new polymorph is of the gagarinite-type (P6₃/m, Z=1) with partially ordered cations, the formula being Na_1.5Y_1.5F₆. This phase is structurally related to the Na_1.5Y_1.5F₆ modification (P₆- , Z=1) stable at ambient conditions. At higher temperatures, the new pressure-induced hexagonal variant of NaYF₄ eventually decomposes into a non-stoichiometric gagarinite-like phase and yttrium fluoride YF₃ (Pnma, Z=4).