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Abstract This study presents the Br‐rich in situ synthesis of blue‐emitting 2D CsPbBr₃nanoplatelets (NPLs) with various Br/Pb ratios using ZnBr₂as a Br precursor to enhance Br ion adsorption significantly. This leads to effective passivation of surface defects, particularly Pb−Br bonds, by increasing the positive charge density around Pb atoms, thus creating a stable bonding environment and reducing defect formation. Consequently, the photoluminescence quantum yield (PLQY) improves from 31.15% for a Br/Pb ratio of 2 to 87.2% for a ratio of 6. NPLs with a Br/Pb ratio of 6 also exhibit longer lifetimes (16.69 ns) and slower bleach recovery dynamics, indicating fewer non‐radiative recombination pathways and effective exciton dynamics. Additionally, NPLs with the Br/Pb ratio of 6 demonstrated better thermal stability, with an activation energy of 124.3 meV, indicating stronger exciton binding. These NPLs also exhibited enhanced stability, with UV tolerance at 43.9% and water resistance at 23.8%, making them suitable for displays and lighting. Furthermore, Br‐passivated CsPbBr₃NPLs are used as blue emitters in prototype white LEDs, achieving a wide color gamut, 126.6% of the National Television Standards Committee and 94.5% of Rec. 2020, demonstrating their potential for high‐quality lighting and advanced display technologies.