Phase transformation between metal halide perovskites serves as a promising route to create new optoelectronic functionalities. Nevertheless, the transformation reported thus far mainly involves the transition between two individual phases (e.g., 0D–3D and 3D–2D), while the transition from one phase to a heterostructure with distinct phases has been rarely disclosed. Here, we report a straightforward one-step procedure to directly convert 0D perovskite to 2D/3D heterostructures and demonstrate the application of the derivatives in random lasing beyond 0D perovskite. This phase transformation was triggered by exposing the 0D perovskite to a water environment and the resultant 2D/3D heterostructure showed much improved water resistance compared to that of its corresponding 3D counterpart as for chemical stability and photoluminescence performance. In addition, we demonstrated that the derived 2D/3D heterostructure readily exhibited random lasing characteristic of incoherent feedback upon optical pumping, while the parent 0D Cs₄PbBr₆ exhibited no sign of light amplification even pumped much harder. The pump threshold and the PL spectral profile were well maintained upon recycling treatment with water, implying the water resistance capability of the developed random lasing. The results suggest that the 2D/3D perovskite composites derived from the phase transformation of their 0D counterpart can serve as promising platforms for nanophotonic applications.