Red emitting phosphors of Mn⁴⁺ doped fluorides are in high demand because the high color purities of phosphors make them potentially useful white light-emitting diodes (WLEDs) as backlights of electronic devices. We herein report an efficient red emitting phosphor KRbGeF₆ : Mn⁴⁺ (KRGF : Mn) that was prepared by an in situ one-pot ion-exchange method at room temperature without organic solvents or HF. Homogenous mixtures of the starting materials with a stoichiometric KRGF : Mn ratio were partly dissolved in various acid solutions, and [MnF₆]²⁻ and [GeF₆]²⁻ successively underwent ion exchange under stirring. More importantly, in situ recrystallization of KRGF : Mn occurred simultaneously in solution with water as a solvent. Evidently, the formation of stable KRGF : Mn crystals propels the processes of dissolution and ion exchange. We also investigated the effect of inorganic acids on the resultant phases and photoluminescence properties of KRGF : Mn. The red phosphor KRGF : Mn prepared in HCl solution shows the highest luminescence intensity with an yield of 80%.