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.