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A narrow-band red emission is a seminal issue for establishing the color-rendering index and color gamut of phosphor-converted white light emitting diode (pc-WLED) applications. In this regard, Mn4+-activated K2SiF6phosphors (referred to as K216) have recently attracted a great deal of attention after their successful commercialization. As with K216 phosphors, Mn4+-activated K3SiF7(referred to as K317) phosphors perform in a manner that is similar to that of K216 phosphors and have been introduced as a narrow-band red phosphor. Despite the acceptable performances of both versions of phosphors, slower decay remains a shortcoming for applications to high-powered LEDs. We introduced K317 derivatives by replacing K with Rb and Cs. As a result, Mn4+-activated (Rb,Cs)3SiF7phosphors were synthesized with different Rb:Cs ratios and exhibited faster decay times by comparison with both the K216 and the K317 phosphors. The present study involved both structural and luminescent investigations along with density functional theory (DFT) calculations for novel (Rb,Cs)3SiF7:Mn4+phosphors. This study reveals the possibility of a completely solid solution with single-phase formation within the entire range of Rb–Cs with retention of the tetragonal structure in the P4/mbmspace group.