Phosphors that exhibit a narrow red emission are particularly interesting due to the advantage of providing a more extensive color gamut and better rendering in LED applications such as displays and solid-state lighting. Although some Eu2+-activated nitridosilicates have been discovered in this regard, K2SiF6:Mn4+ phosphors are the only option in actual LED applications thus far. We discovered a novel phosphor, K3SiF7:Mn4+, with P4/mbm symmetry. The luminescent properties of K3SiF7:Mn4+ are almost identical to those of the K2SiF6:Mn4+ phosphor, but its materials identity is distinct due to a completely different crystallographic structure, which leads to reduced decay time. The fast decay is one of the most serious disadvantages of existing K2SiF6:Mn4+ phosphors. The K3SiF7:Mn4+ phosphor was examined in comparison to the K2SiF6:Mn4+ via density functional theory calculation, Rietveld refinement, X-ray photoelectron spectroscopy, X-ray absorption near-edge structure spectroscopy, and time-resolved photoluminescence.