<p>Fluoride red phosphors have been extensively applied in WLEDs due to their broadband excitation and narrowband emission, but the low luminescence efficiency and poor stability severely restrict their practical applications. Herein, Nb⁵⁺ ions are doped into the Ta⁵⁺ sites in Mn⁴⁺-activated K₂TaF₇ to optimize luminescence efficiency and stability. X-ray diffraction analysis confirms that the K₂Ta_1−<i>x</i>Nb_<i>x</i>F₇:Mn⁴⁺ solid solutions with monoclinic structure are successfully synthesized. Scanning electron microscopy result reveals that the phosphors consist of irregular rod-like crystals with average length of 20–50 μm and width of 2–10 μm. Due to the distorted octahedral environment around Mn⁴⁺, these Mn⁴⁺-activated phosphors exhibit distinct zero phonon line in their emission spectra. The Mn⁴⁺ rare particle surface, improved structural rigidity together with low solubility of the solid solutions lead to the inhibited hydrolysis of [MnF₆]^2‒ in water and hence restrain the degradation in emission intensity, resulting in the simultaneous achievement of superior luminescence efficiency and moisture resistance. A warm WLED device with a low correlated color temperature of 3877 K and a high color rendering index of 90.6 is obtained based on K₂Ta_0.7Nb_0.3F₇:Mn⁴⁺ phosphor immersed in water for 1440 min. This work demonstrates the promising potential of K₂Ta_1−<i>x</i>Nb_<i>x</i>F₇:Mn⁴⁺ as efficient red phosphors for advanced lighting applications.</p>