LAUSR

All‐inorganic bismuth‐halide perovskites are promising alternatives for lead halide perovskites due to their admirable chemical stability and optoelectronic properties; however, these materials deliver inferior photoluminescence (PL) properties, severely hindering their prospects in lighting applications. Here, a novel air‐stable but non‐emissive perovskite Rb3BiCl6 is synthesized, and the material is used as a prototype to uncover origin of the poor optical performance in bismuth‐halide perovskite. It is found that the extremely strong exciton–phonon interactions with a large coupling constant up to 693 meV leads to the seriously nonradiative recombination, which, however, can be effectively suppressed to 347 meV by introducing Sb3+ ions. As a result, Sb3+‐doped Rb3BiCl6 exhibits a stable yellow emission with unprecedented PL quantum yield up to 33.6% from self‐trapped excitons. Systematic spectroscopic characterizations and theoretical calculations are carried out to unveil the intriguing photophysical mechanisms. This work reveals the effect of exciton–phonon interaction, that is often underemphasized, on a material's photophysical properties.