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The development of metal‐halide perovskite nanocrystals (NCs) that yield bright and stable emission is of great importance. Previous reported perovskite NCs are mostly based on APbX3‐type family fabricated via ligand‐ or surfactant‐assisted chemical approaches. However, realizing bright and stable emission remains a challenge because of desorption of ligands/surfactants during long‐term operation. Herein, Ruddlesden–Popper (RP)‐type (A)2(MA)n‐1PbnBr3n+1 NCs with size less than Bohr radius stabilized in mesoporous silica scaffold, which are prepared in situ via physical approach at low temperature are introduced. The RP NCs in mesoporous silica exhibit the formation of spatially and electronically separated quantum wells, efficient energy funneling between different n phases for bright emission (photoluminescence quantum yields of ≈99%), high irradiation stability (T70 = 110 days), and long‐term stability (T90 = 110 days). These RP NCs have broad potential for bright light‐emitting diodes, high‐resolution PL imaging, and waterproof inks. Importantly, for the first time, stretchable perovskite X‐ray scintillator is demonstrated with excellent X‐ray imaging with resolution greater than 14 line pairs mm−1. These findings offer a paradigm to motivate future research toward stable and efficient perovskite optoelectronics. [ABSTRACT FROM AUTHOR]