Colloidal organic–inorganic lead halide perovskites have gained tremendous attention due to their excellent optical properties and promising future in optoelectronic applications. However, the lack of large scale synthesis methods and structural instability in moisture conditions restrict their practical applications. We report, for the first time, a gram-scale aqueous synthesis of MAPbBr₃ (MA = CH₃NH₃⁺) using PbCO₃, bottom-up synthesis of MAPbBr₃ NCs and in situ bottom-up synthesis of MAPbBr₃@poly(3-bromothiophene) nanocomposite. The synthesized nanocomposite material exhibits bright green fluorescence and excellent water-resistivity. Together, the polymer-based bottom-up and in situ encapsulation strategy of MAPbBr₃ NCs will pave the way for synthesizing other lead halide water-resistant perovskites that will be used in thin-film based light-emitting devices.