We successfully prepared a red-emitting phosphor Rb₂NbOF₅ doped with Mn⁴⁺, and investigated its crystal structure, luminescent properties, and applications in detail. Mn⁴⁺ ions in Rb₂NbOF₅ exhibit an intense red emission with high color purity upon a broad excitation in the blue region. The related crystal-field parameters of Mn⁴⁺ in Rb₂NbOF₅ have been estimated according to the luminescent spectra. More importantly, Rb₂NbOF₅:Mn⁴⁺ has high thermal-quenching resistance and color stability. We further fabricated a warm white light-emitting diode using the phosphor, emitting intense white light with a high color-rendering index of 90 and a low color temperature of 3418 K. The results indicate that Rb₂NbOF₅:Mn⁴⁺ is a potential red-emitting phosphor for white light-emitting diodes.