Materials emitting red light (∼611 nm) under excitation with blue light (440–470 nm) are highly desired for fabricating high-performance white light-emitting diodes (LEDs). Conventionally used red light-emitting materials (e.g., Y₂O₃:Eu³⁺ or Y₂O₂S:Eu³⁺) exhibit a relatively poor blue light-absorption and a weak chemical stability. In this paper, we reported on a novel red light-emitting material based on a (K_0.5Na_0.5)NbO₃ (KNN) matrix co-doped with Bi³⁺ and Eu³⁺ showing a strong absorption in the blue light region and superior water resistance properties. The crystal structure, photoluminescence, thermal stability, energy transfer mechanism and water resistance behavior of the samples were systematically investigated. A strongly enhanced red light-emission at 616 nm originating from the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions was observed after adding Bi³⁺ ions as an alternative to increasing the Eu³⁺ concentration due to the energy transfer from Bi³⁺ to Eu³⁺. After adding 0.05 mol of Bi³⁺ as sensitizer, the sample with the composition of (K_0.5Na_0.5)_0.90Eu_0.05Bi_0.05NbO₃ exhibited the strongest red light-emission and a high quantum yield under 465 nm excitation. Doping with Bi³⁺ also endowed the KNN:Eu³⁺ samples with a good thermal stability (83% of the initial intensity at 150 °C) and a superior water resistance behavior (94.3% of the initial intensity after 40 h of immersion). These results demonstrate the great potential of the Bi³⁺/Eu³⁺ co-doped KNN material for a future application in white LEDs and novel multifunctional devices.