A new red-emitting phosphor based on polyfluoride Na₃Li₃Sc₂F₁₂ doped with Mn⁴⁺ was prepared via a co-precipitation method at room temperature. Its crystal structure, luminescence properties and applications are discussed in detail. The as-synthesized phosphor emits red light with intense zero-phonon line (ZPL) at 618 nm under the excitation of 461 nm light. A higher energy of the ²E_g level of Mn⁴⁺ was observed in Na₃Li₃Sc₂F₁₂:Mn⁴⁺. The Racah parameters B and C were calculated to be 604 cm⁻¹ and 3840 cm⁻¹, respectively. The as-obtained Na₃Li₃Sc₂F₁₂:Mn⁴⁺ (4.86 mol%) shows a high internal quantum efficiency (IQE) of 66.8% and better resistance to thermal quenching. A white light-emitting diode (w-LED) with a high color rendering index of 92 and a low color temperature of 3694 K was fabricated. Hence, Na₃Li₃Sc₂F₁₂:Mn⁴⁺ as a new red-emitting phosphor has great potential application in w-LEDs.