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Red-emitting materials in the 615–620 nm range are essential for high-performance displays, providing the saturated red required by Rec. 2020 standards. Most Mn2+-based materials emit light above 650 nm, rendering them unsuitable for display applications. Here we report a fully inorganic, lead-free compound, K4MnCl6, that exhibits a bright red emission centered at 615 nm with a relatively high photoluminescence quantum yield (PLQY) of 80% among Mn2+-based inorganic PIMs. Density functional theory (DFT) calculations reveal that the red emission arises from a Jahn–Teller-distorted, vibronically allowed 4 T1 → 6 A1 transition of the Mn2+ ions. K4MnCl6 exhibits superior thermal quenching resistance and moisture stability compared to CsMnCl3 and Cs2MnCl4. This is due to its high Debye temperature, suppressed thermal ionization, and hydrophobic K+ layers. Structural and electronic analyses link reduced Mn2+–Mn2+ spacing and low dimensionality to efficient energy transfer. Furthermore, UV-excited red-emitting light-emitting diode (LED) based on K4MnCl6 achieves 691.4 cd/ m2 at 7.0 V with stable emission, and ≈3 cm link patterns fabricated via doctor-blade printing glow vividly under UV light, highlighting its potential in display and photonic applications.