Abstract Ternary indium (In)‐based metal halides are gaining increased attention as promising lead (Pb)‐free alternatives for light‐emitting diodes (LEDs) owing to their broadband emission and high photoluminescence quantum yields (PLQYs). However, most of the pure In‐based compositions suffer from instability against moisture and thermal stress, leading to a rapid degradation of material and hence their optical properties. Herein, a new kind of organic–inorganic hybrid In‐based metal halide BA 6 InCl 9 (BA + = C 4 H 11 N + ) is presented, which exhibits stable crystal structure and material composition at both ambient (over 5 months) and heating conditions (up to 200 °C). Besides, the Huang‐Rhys factor of ≈4.94 determined for BA 6 InCl 9 is considerably smaller than most Pb‐free perovskites, which suggests a relatively weak exciton‐phonon coupling in these crystals. By trace amounts of antimony (III) (Sb 3+ ) doping, the PLQY of the BA 6 InCl 9 single crystals can be markedly improved from ≈25% to 95%, and the ensuing down‐conversion LEDs exhibit bright orange‐red emission with an external quantum efficiency of 0.12%, which retains ≈50% of the initial luminance after 380 min of continuous operation in ambient air. It is forseen that the study will prompt future research on In‐based metal halides and their use in stable light‐emitting applications.