<p>Halide perovskites are promising solution-processable optoelectronic materials. Traditionally, their synthesis has relied on coordinating solvents or molecules to dissolve perovskite precursors. Such reliance dictates solvent choice, constraining perovskite fabrication techniques and perovskite material quality and raising safety and environmental concerns for scale-up fabrication. Here we demonstrate that solvent-in-salt (SIS) systems, composed of water or alcohol and common halide salts, serve as an alternative medium for synthesizing diverse perovskite compositions. These SIS systems eliminate reliance on coordinating solvents, with electrostatic screening induced by highly concentrated ions identified as the underlying mechanism for perovskite precursor dissolution. We show that the SIS systems are room-temperature analogues to molten-salt mediums widely used for inorganic materials synthesis, and the as-grown perovskite single crystals exhibited improved charge carrier transporting properties comparable to that of molten-grown inorganic semiconductors. These SIS systems are green, safe, stable and scalable.</p><p></p>