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Blue-light-emitting and quantum-confined two-dimensional CsPbBr3 perovskite nanoplatelets (NPLs) are viable candidates for blue-light-emitting diodes compared to mixed-halide CsPb(Br/Cl)3 nanocrystals that exhibit segregation of halide ions. Unfortunately, the NPLs suffer from poor color stability and insufficient colloidal stability. However, the improvement in their stability has not been extensively studied. In this study, the blue emission of CsPbBr3 NPLs is stabilized by B-site doping approach, wherein a few of Pb2+ octahedral sites are replaced with small-sized Cu2+ ions. Cu2+-doped NPLs exhibit greater long-term stability in ambient conditions and better photostability compared to their undoped counterparts. First-principles calculations reveal that the Jahn–Teller distortion of (CuBr6)4− octahedra results in the shorter Pb-Br bonds and contraction of the entire CsPbBr3 lattice, which in turn increases the band gap of the CsPbBr3 NPLs. Cu2+ dopants also reduce the surface energy of the NPLs and impart highly desirable long-term stability. Therefore, the outcomes of this work might be a step forward towards improving the stability of two-dimensional perovskite NPLs.