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State-of-the-art cesium-containing multiple-cation perovskites are generally synthesized from stock solutions of perovskite precursors and CsI in DMSO and DMF. However, compositional diversity of multi-component perovskites is significantly hampered due to the poor solubility of other cesium halides in these solvents. Here, we show how insoluble CsCl, as a new source of cesium cation, can be integrated into a multiple-cation perovskite material by a one-step method involving grinding of the precursors. The resulting polycrystalline powder is fully soluble in a DMSO/DMF mixture and allows formation of perovskite thin films. ¹³³Cs solid-state MAS NMR data indicate that the cesium cation is almost fully (90%) incorporated into the 3D perovskite lattice, while the remaining 10% forms a cesium-rich mixed-halide secondary phase. The planar heterojunction device fabricated using this original mechanoperovskite …