We have developed a facile low-temperature synthetic method for the preparation of NaMnF3 nanocubes with Er3+ and Yb3+ ions homogeneously incorporated in the host lattice. The effects of the reaction temperature, and the volume ratio between ethanol and DI water on the morphology of NaMnF3 nanocubes are systematically investigated. The NaMnF3 nanocubes can be produced in the low temperature range (25–80 °C), and the higher reaction temperature (80 °C) is favorable for the formation of a smooth surface. The formation of NaMnF3 nanocubes strongly depends on the ethanol solvent. The morphology and single-phase of the obtained samples could be well maintained by controlling the doping concentration (Yb3+ ≤ 20 mol%). Single-band red upconversion emission can be generated in Er3+/Yb3+ codoped NaMnF3 nanocubes due to the energy transfer between host Mn2+ and dopant Er3+ ions. It is revealed that our NaMnF3:Er3+/Yb3+ nanocubes irradiate the brightest red luminescence at the dopant concentrations of Er3+ (2 mol%) and Yb3+ (15 mol%), which is stronger than that of the hexagonal-phase NaYF4:Er3+/Yb3+ phosphor.