The feasibility of coating K2SiF6:Mn4+ phosphor particles with an Al2O3 layer, in order to enhance the optical properties and improve the chemical and thermal stability, has been studied. Two types of K2SiF6:Mn4+ phosphor particles have been coated with a thin (3-25 nm) Al2O3 layer using atomic layer deposition in a fluidized bed reactor. The Al2O3 coating layer does not have any significant effect on the spectral excitation and emission features, but the emission intensity of conventional K2SiF6:Mn4+ (KSF-1) decreases, which is ascribed to the formation of undesirable MnO2. The thermal quenching of the KSF-1 phosphor in an inert atmosphere is reduced by the Al2O3 coating layer. Degradation during the deposition of Al2O3 is prevented by using K2SiF6:Mn4+ particles with an undoped K2SiF6 shell (KSF-2). The Al2O3 coating layer has a positive effect on the stability of both the KSF-1 and KSF-2 phosphors in a water environment, as the Al2O3 layer acts as a barrier against the hydrolysis of K2SiF6. In air, however, water present in the Al2O3 coating layer enhances the degradation of the phosphor at elevated temperatures.